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Suppose there is a species similar to humans, but with the difference that their eyes change color based on their emotions. For example they would turn red to signal anger, pink for lust, rust for rage, black for gloom and so on.
In this type of setup: Is it possible to lie undetected?
What you've got there is a slightly more refined and biologically pre-installed version of a polygraph machine, and it has all the problems inherent in a polygraph. There's a silly number of people who have taken a polygraph test and gotten a false positive or false negative.
The thing you're measuring is essentially the emotional response to the question and the response... Given the question "did you kill this person?" the subject may exhibit:
Emotions are an unreliable indication of guilt, far too easy to control and far too easy to misattribute.
Edit: Something I forgot to mention...
The biggest fault of the idea of a lie detector is the same as the biggest fault of human witnesses in the first place. Specifically, even a perfectly accurate lie detector can only tell if the person saying what they believe the truth to be.
A person who has sufficient belief in the truth of their statement will pass any lie test you can dream up, regardless of how wrong their statement is, simply because it is true according to their version of reality. After all, everyone has their own version of reality based on how their experiences and how they processed those experiences Subjective reality is the source of many faulty eye witness reports. A person's emotion response is to what they believe not to what is true.
You cannot feel guilt for something you believe you aren't responsible for. You can't feel grief for something you believe you haven't lost. For the human mind, truth is subjective and reality always loses to belief.
You could make a religion out of that...
In this setup lying will not change the color. My only point is that it will be harder to lie and I want to know the loopholes so that I can get my story straight
Well, I can think of a fairly big loophole. Their eyes may show their emotions, but there's nothing stopping them from lying about why they're feeling that particular emotion. It's a specific circumstance, but it's still a circumstance in which they can lie.
Consider a detective investigating a murder. He's talking to one of the victim's friends and notices that his eyes keep flashing rust-red for anger. He inquires about it, and the man explains that he's angry with the police for having not found the murderer yet. In reality, he is the murderer - he and the victim had had a falling-out. But for now at least, he's covered that up with a perfectly plausible lie.
In such a culture it might be normal to hide your eyes so that you would normally not show your intentions to others. If this is the norm than the eyes are no problem in normal everyday conversations. It would be difficult if you knew someone really well and wouldn't hide your eyes normally. That would be extremely suspicious and it wouldn't help if you were for example interrogated and forced to show your eyes.
Lying is one of those situations that quite often arise. The little examples are "Have a good day!" when you really want to say "Just leave already!". Just like people in our society learn to control their voice and keep their emotions enough under control to not necessarily show their intentions these people would learn how to keep the signals they are sending with their eyes to a minimum.
It would take a lot of time and effort, but the skilled people could tell eyes and just keep at a relaxed, peaceful green - because they learned to think about something else while telling their lie. It would be difficult and probably not possible for a lot of normal people, but that's not too different from our society where some people are naturally good at lying, some people are exceptionally good at learning to lie systematically and some people can be read like an open book.
It's just one more signal that shows that you are lying - like little tics where you touch your necklace or wrist watch, twitch your eyes, look to the upper left corner of your eye, dart your eyes around, start sweating more, turn away slightly, ...
Telling a lie is not an emotion. Your response and feelings about telling the lie, however, are.
If you are able to lie without getting emotionally involved or conflicted, then yes, the lie can go undetected by any emotional detection system.
Your mind/body first has to detect that it is a lie, then it has to react to your telling a lie, then it has to apply an emotional tag to it. If you have learned not to get emotional about lieing (some people can) so it becomes routine, then there is no emotional component. For instance, you have to believe that lieing is wrong, in order to generate a negative emotional response.
However, there is a difference in the brain function between telling a lie and telling the truth. With a lie, the brain has to CREATE the response. When telling the truth, the mind RETRIEVES the memory. Different pathways.
In a 'yes' or 'no' answer, the body first compares the two (question and memory recall) and makes a sub-conscious decision. This generates a tag, which then is translated into your answer. It is normally an internal dissonant result that creates the emotional response in humans, when their response does not match the result of the comparison. That is, your mind first determines (behind the scenes) if the two are the same or different, and then in a separate process it generates the outward verbal response. The mind will know if they are dissonant or not. So if your detection system works on this dissonance, and not just emotion, then yes it can detect a lie.
So in addition to the answers previously mentioned if the person intentionally brought to mind emotions.
So if I were saying I am not attracted to X and held in my mind someone who I was not attracted to I would not be feeling an emotion so the person reading my eyes might be fooled.
This is not so different from reading micro expressions on the faces of real people ( https://www.psychologytoday.com/blog/deception/200904/micro-expressions-and-good-liar ) . There are techniques that are useful in decieving other people and techniuqes useful in reading past the techniques that people use. Your question adds another factor and if this is a rare lone case people questioning would not be trained the read the eyes.
One aspect of coloring that isn't mentioned in other answers is the ability to discern the correct color.
You can change the appearance of a color by changing the color of surrounding or adjacent colors. The link below is a small sample of optical illusions that can change the perceived color of an object.
There are many other illusions to be Googled that show more than just a black/grey/white color illusion like many are on the linked page, so it's not just those colors involved.
Also, men and women tend to see colors differently.
In color experiments the men and women tended to ascribe different shades to the same objects.
https://news.nationalgeographic.com/news/2012/09/120907-men-women-see-differently-science-health-vision-sex/
Men also have a higher possibility of being color blind.
Men are much more likely to be colorblind than women because the genes responsible for the most common, inherited color blindness are on the X chromosome. https://nei.nih.gov/health/color_blindness/facts_about
On a less scientific side of things, my mom (graphic designer and general artist) will tell me about 30 shades of red by name, while I (programmer and non-artist) will see about 15 shades of red and call them all red. So that tells me that you can be trained to determine the shades of colors, given your eyes are up to the challenge (non-colorblind).
Back to objective matters, colors on different media as well as different lighting can change the color of something. When I do image editing, an image might appear green on one of my monitors, but be white on another monitor. Something that looks white outside at noon might look yellow indoors under fluorescent lighting.
https://superuser.com/questions/127533/colors-differ-in-the-dual-monitor
https://en.wikipedia.org/wiki/Color_temperature
Besides what I've mentioned, there's a whole range of color representations that can cause you problems when discerning colors, and they all boil down to color space:
https://en.wikipedia.org/wiki/Color_space
Colors are great, but in my experience in trying to use colors, don't try to name them or nail them down to "one thing." (Is that rust or burnt sienna? (It's f'n red!)) That only leads to arguments, headaches, and exasperation. ;-)
Since it's something I've done a fair amount of, I immediately thought of acting (emphasis mine):
Goldstein looks at three categories—pretense, lying, and acting—as they fit into a trio of cognitive parameters. First, what is being presented perceptually and if it is actually happening or is just pretend; second, what behavior is being shown and whether that behavior is a cue to reality; and finally, whether the exhibited behavior is intended to fool the audience. On the first parameter, Goldstein says, all three categories are in agreement. In the cases of pretense, lying, and acting, “what is being presented perceptually, what we’re seeing, is not real.”
In the second parameter, there is some variation among the categories. “In pretense, the behavior is a cue to the fact that what [someone] is doing is not real. You’re smiling even though you say you’re sad, or you’re not using a cup when you pretend to drink,” Goldstein explains. “In deception and acting, though, the behavior [alone] is not a cue to the fact that what you’re doing is not real.”
The final category is the trickiest of all: Are actors trying to make people believe that what they’re doing is true? Well, yes and no. Acting is not lying and neither is it pretense, but both flirt with what is “true” or real to varying degrees.
particularly modern techniques of it:
To reach this "believable truth", Stanislavsky first employed methods such as "emotional memory." To prepare for a role that involves fear, the actor must remember something frightening, and attempt to act the part in the emotional space of that fear they once felt. Stanislavsky believed that an actor needed to take his or her own personality onto the stage when they began to play a character. This was a clear break from previous modes of acting that held that the actor's job was to become the character and leave their own emotions behind. Later Stanislavsky concerned himself with the creation of physical entries into these emotional states, believing that the repetition of certain acts and exercises could bridge the gap between life on and off the stage.
as well as therapeutic techniques of psychology:
. . . we often assume that whatever we think or feel must be true simply because we thought or felt it. Nothing could be farther from the truth, as Sarah learned. In many cases, we are actually looking at life through the lens of deceptive brain messages [and] seeing our circumstances, other people or ourselves from a distorted and inaccurate viewpoint. It's only when we are able to identify and dismiss the faulty logic of deceptive brain messages and believe in ourselves that we begin to break free and change our behaviors so that they align with our true goals and values in life.
bearing in mind that we are positing a person whose actual intention ("true goal") is to deceive, and that there is nothing inherently preventing the aforementioned techniques from being put to that purpose:
someone trained to maintain confidence in their feelings, resist emotional manipulation, and 'summon' emotional memories of their choice can lie effectively, (barring flawless mind-reading - and eye-color change based on present emotion isn't flawless).
Relatively simple answer:
Probably in the form of either and eyedrop to change the color temporarily, or a hormone that causes / trickes eyes to think that the being is feeling a particular emotion.
This option comes in a veriety of flavors for your world:
These drugs can be advanced, or they could come from a plant.
They can have side effects
There can be different qualities and types
They could be made illegal (and in turn create a black market)
A person cought with the drug would probably be considered untrustworthy, and would lose his reputation
Other people can be framed, causing them to lose their trust and reputation
And, as @cobaltduck suggested in the comments, colored eye contacts would work as well. Beings found with them would also be no longer trusted, can be illegal, and have different qualities, etc.
Being "caught" on a lie because your emotions are known doesn't take out the effect of words.
You're angry at your wife because she embarrassed you at a party.
"That dress makes you look fat as a hog, but I guess you already knew that because your diet clearly isn't working."
You're lustful for your wife and you're trying to get the foreplay going.
"You look so sexy in that dress, and all the other men at the party were checking you out."
In either of those situations, from the point of view of the other person, would it matter if those statements were technically lies?
Humans already display their emotions through a number of tells, such as facial expressions and body language and vocal pitch. Most of the time, and for most people, people are not trained to exert great control over these tells, and so our emotions are not especially challenging to read if one cares enough.
And yet telling lies is still commonplace.
Can a person lie if his eyes tell his emotions?
Yes.
For example, I'm angry at someone at work and want them to feel bad about something, so I tell a lie about some of the work they did.
"Your proposal was complete garbage. I know why you failed college."
"Why are you so angry?"
"Because my neighbor's teenage kid was speeding through the street and killed my dog this morning."
It's obvious that the original speaker is angry because of eye color, but it's not obvious which one of those statements are lies.
Any combination of those options could be lies or not lies, and yet it wouldn't change the emotion or effect of the speaker's words.
Assuming there was an accurate way to assess lying with the color of the eyes indicating it is a lie.
Well, is it a lie if it's technically truthful?
Consider the following scene from the Comic Book story "52" from DC:
During the course of the series, Superman has not been active because he lost his powers just prior to the story's beginning. A new hero named Supernova has appeared in Metropolis and Clark Kent landed the first interview with him. Lex Luthor thinks that Superman is merely hiding and waiting for Luthor to return to his old tricks, so he has Clark Kent kidnapped and drugged with a potent truth serum (and yes, it works). The the following (paraphrase) conversation occurs:
Lex: Who is Supernova? Clark: I don't know. Lex: Did he seem familiar? Clark: A little, but I can't place him. Lex: He reminds you of Superman, doesn't he. Clark: Laughs in Lex's face I could tell from our meeting that Supernova is definitely not Superman.
Now, keep in mind, Clark doesn't lie at any of these points... but he doesn't clarify anything about the lie. Lex Luthor wants to know who Supernova is, but Clark doesn't know, though does admit he seems familiar (I'd spoil the story by revealing, but Clark met Supernova's unmasked personality a while back. Luthor would also know that since he covered some heroes and superman stories, a flying caped man in a mask could seem familiar to Clark, given his work.). Lex then asks if Supernova reminds Clark of Superman.
Now, for the uninitiated, Clark Kent has some very good reasons as to why he knows that Supernova is not Superman, but doesn't want to tell Luthor. So his response is given in a truthful statement. Clark can truthfully say that having met, he could tell that Superman is not Supernova. After all, he's gotten to know Superman quite well over the years of covering him.
So have your character tell the truth... but in a way that is deceitful.
Take another famous moment in real life history: "I did not have sexual relations with that woman."
Legally that is a true statement... under Washington D.C. laws at the time what was alleged to have occur between the speaker and "that Woman" was not part of the legal definition of "sexual relations". The speaker would have known this, but was hoping that the people hearing him say that would would consider the specific allegation under the common term "sexual relations".
As a final note, when writing this character, the proper frame of mine is reflected in this wonderful conversation from Deep Space 9:
Brashir: But which of the stories [about Garak's banishment] is true. Garak: My dear doctor, they're all true. Brashir: Even the lies? Garak: Especially the lies.
Since posting my last answer, I have come across further research which is perhaps germane to the answer. The OP has not really clarified if we are to look at this as an alien emotional response, or human emotional response. If it is an alien emotional response, then the question is not answerable without further knowledge of the alien emotional response.
If it is with respect to human functioning then the following is pertinent. I referenced it in a comment, but I am now referencing it in an answer. Humans do not change their eye color, but they do detectably change their pupil size. See Eye-Opener: Why Do Pupils Dilate in Response to Emotional States?
"Pupil dilation can betray an individual's decision before it is openly revealed," concluded a 2010 study led by Wolfgang Einhäuser-Treyer, a neurophysicist at Philipps University Marburg in Germany. Participants were told to press a button at any point during a 10-second interval, and their pupil sizes correlated with the timing of their decisions. Dilation began about one second before they pressed the button and peaked one to two seconds after.
and
For more than a century scientists have known that our eyes' pupils respond to more than changes in light. They also betray mental and emotional commotion. In fact, pupil dilation correlates with arousal so consistently that researchers use pupil size, or pupillometry, to investigate a wide range of psychological phenomena. And they do this without knowing exactly why our eyes behave this way.
So it is not far-fetched to posit that eye color could also change with emotional changes. The evolutionary purpose of it would be hard to posit, however the purpose for a great deal of evolutionary changes in earth-bound species is hard to fathom.
I found the notion developed in this article that pupil size reliably, predictably, and autonomically changes with cognitive effort particularly intriguing. Lying takes greater cognitive involvement than telling the truth. The lie has to be fabricated. So add this factor to eye color change with emotion, and the resulting effects could be interesting.
But I also mentioned in a comment elsewhere herein, that the cultural zeitgeist would also have to be considered. In Western justice systems, we superficially believe in the basic tenant of law that a person is innocent until proven guilty. Under such conditions, it is difficult to prove guilt in a lie detector test. The bias is towards innocence. It is thus harder to prove someone is lying.
But in a culture that believes in guilty unless proven innocent, once charged, the burden of proof rests with the accused. They would have to prove they were NOT lying. Under such a system, any sign of lying would be sufficient proof of lying. Sort of like the inquisitions of old. So any world built on such species would have to clarify the cultural and legal mores around such a trait.
COULD it be used is a very different question than WOULD it be used and generally be accepted as proof of lying. This would be a very interesting plot line to develop.
