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Ever since reading through the creative answers in this question

(How can I explain alien skin being different colors?)

I've been thinking about the logistics behind fantastical skin colors. However, one part that stuck out to me was the idea that pH levels could influence a creatures outward appearance.

One of the more promising things I found relating to that was bromothymol blue. Bromothymol blue is a pH indicator that has a range of colorless to bright blue, but with human blood levels would likely appear between tealy-green to muted turquoise. It is usually used to test amounts of carbonic acid, an element that already exists to some extent in human blood.

This leads me to main questions:

  • How can a humanoid creature maintain this pH indicator in its physiology (as in, not immediately flushing it out of the system or dying of exposure)?
  • How can I make the effects of the bromothymol blue manifest on the skin?
  • As a bonus, are there any other chemicals that could work in a similar way and not require too many changes to human biology?
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What worries me about the bromothymol blue proposal is that it doesn't appear to be an organic chemical, i.e. no living organism produces it. This means that it's not possible (or at least unlikely) to be created in nature, and it would have to be artificially replenished. Those are obviously some big hurdles to overcome.

There are naturally-occurring pH indicators that serve just the way bromomythol blue does, for your purposes.

  • Anthocyanins are synthesized in plants through the phenylpropanoid metabolic pathway in certain plants, using a number of enzymes. For this synthesis to work, an organism would in turn need to synthesize the enzymes used in this pathway. This is often a bit of a problem when it comes to introducing new chemicals into an organism: There are way more intermediate enzymes and proteins that are needed, and a delicate balance must be maintained. Still, it's not an impossible task - just a very difficult one. Anthocyanins are non-toxic to humans, so it shouldn't be a danger to your organisms.
  • In Hydrangea macrophylla, the color of the flower is determined by levels of aluminum in the soil. Without aluminum, red anthocyanin dominates; when some sort of water-soluble aluminum compound is added, it can bind with the anthocyanin and turn the flower blue. To harness this, you'd want to look and anthocyanin production again, and determine how aluminum could get to the organism's cells (I'm assuming it would come through diet). Once again, you probably won't have toxicity issues.
  • Litmus is the result of naturally-occurring compounds found in lichens. Unfortunately, it's a complex mixture when it works the best, but its properties are mainly due to a chemical called 7-hydroxphenoxazone, based in part on phenoxazine. I don't know whether a single organism could produce all of the constituent compounds.

I imagine that the main production centers of whichever indicator you choose would be near the top layers of the skin. Currently, special cells called melanocytes create melanin, which determines skin color. All you have to do is swap out melanocytes for whatever modified cells you design to create the pigments of your choice. They'll reproduce the pathways found in plant cells to produce the indicators, but that doesn't mean that they'll have all of the same features (e.g. cell walls) of plant cells. They'll be hybrids, really.

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  • $\begingroup$ I genuinely never knew that was the reason for hydrangea colouration!! I thought pink/blue flowers were just different strains of the plant. You learn something new every day. $\endgroup$ – Joe Bloggs Oct 9 '17 at 6:57
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There are people with blue skin in real-life. This has something to do with hemoglobin.

A blue skin tone can be caused by methemoglobinemia where an excess of methemoglobin (a form of haemoglobin that contains ferric iron rather and ferrous iron and is useless or very poor at carrying oxygen in the blood) builds up. This can occur if there is a deficiency in the enzyme called cytochrome-b5 methemoglobin reductase which converts methemoglobin into haemoglobin.

Normally, people have less than 1 percent of methemoglobin. Between 1 and 10 percent does not have much effect and levels greater than 20 percent tends to cause heart abnormalities, seizures and even death. But at levels of between 10 and 20 percent a person can develop blue skin without any other symptoms.

Incidences of methemoglobinemia are extremely rare, but do occur and one family in particular is on record as having been seriously affected.

Martin Fugate came to Troublesome Creek in eastern Kentucky from France in 1820 and family folklore says he was blue. He married Elizabeth Smith, who also carried the recessive gene. Of their seven children, four were reported to be blue.

There were no railroads and few roads outside the region, so the community remained small and isolated. The Fugates married other Fugate cousins and families who lived nearby, with names like Combs, Smith, Ritchie and Stacy.

Benjamin Stacy, born in 1975, is the last known descendent of the Fugates to have been born exhibiting the characteristic blue color of the disease, but lost his blue skin tone as he grew older. Benjamin "Benjy" Stacy so frightened maternity doctors with the color of his skin "as Blue as Lake Louise" that he was rushed just hours after his birth in 1975 to University of Kentucky Medical Center.

As a transfusion was being readied, the baby's grandmother suggested to doctors that he looked like the "blue Fugates of Troublesome Creek." Relatives described the boy's great-grandmother Luna Fugate as "blue all over," and "the bluest woman I ever saw."

The most detailed account, "Blue People of Troublesome Creek," was published in 1982 by the University of Indiana's Cathy Trost, who described Benjy's skin as "almost purple."

The disorder can be inherited, as was the case with the Fugate family, or caused by exposure to certain drugs and chemicals such as anesthetic drugs like benzocaine and xylocaine. The carcinogen benzene and nitrites used as meat additives can also be culprits, as well as certain antibiotics, including dapsone and chloroquine.

Dr. Ayalew Tefferi, a haematologist from Minnesota's Mayo Clinic. Is reported to have described methemoglobinemia patients' lips as being purple, their skin as being blue and their blood as "chocolate colored" because it is not oxygenated.

See also https://www.livescience.com/34410-blue-skin-argyria-methemoglobinemia.html

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    $\begingroup$ Hi, Welcome to Worldbuilding! +1 for a very interesting article! However, owing to the nature of the internet, links will occasionally go dead, making them no longer (easily) accessible, so would you be able to edit your answer and summarise/quote from the article the relevant bits of information? Thanks $\endgroup$ – Mithrandir24601 Oct 7 '17 at 10:34
  • $\begingroup$ Yes well worth including further details. I will maked an edited version of this for proposed inclusion here if agreed. $\endgroup$ – Slarty Oct 7 '17 at 13:43
  • $\begingroup$ I can vouch for how disconcerting it can be to see a blue baby, although not for the same reasons described here. When my son was born he was also blue/purple (I suspect not uncommon due to low levels of oxygen during birth). Fortunately immediately after his first breath his whole body turned pink, after the second breath his arms and legs turned pink and shortly after that his hands and feet followed suit. $\endgroup$ – Slarty Oct 7 '17 at 15:36
  • $\begingroup$ It's been advanced that your answer is largely pasted verbatum from other pages, yet presented as your own. I just made the whole thing indicated as quoted, rather than delete as plagiarism. You should edit the post to indicate what parts are excerpted from where, and consider how detailed you need to get. $\endgroup$ – JDługosz Oct 7 '17 at 23:13
  • $\begingroup$ @JDługosz Actually I simply linked an article in my one sentence answer. Thanks for editing (or whomever included the article text) it to include article information, but I take offense that you are accusing me of plagiarism. I mean, unless posting a one liner to an article that I clearly didn't write and never claimed to have written is plagiarism, then guilty as charged I guess. $\endgroup$ – Spencer Oct 9 '17 at 23:03
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It depends on what level of change you are willing to accept to the humanoids biochemistry skin colour changes are “easy” to achieve in nature. By easy I don’t mean simple I mean that there is sufficient biochemical variety available in nature to produce almost any coloured effect you want given sufficient evolutionary pressure and time.

The Octopus is one case in point as can be seen in the videos here: video1 video2 If you specifically want to use bromothymol blue then I doubt this could be achieved without very extensive reworking of human biochemistry, although I suspect it would be possible.

One good human friendly option might be to take something that works in a vaguely similar way to haemoglobin which has a red – blue colour change based on oxygen and carbon dioxide balance. If the venous blood and arterial blood were both carried close to the surface the body could more easily switch between the two in a process similar to blushing. Another option would be to include hemocyanin in the blood as used by the octopus but this would involve considerable metabolic rearrangement.

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