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Human-made space-travelling objects seem to follow certain colour palettes. For example the interiors of human space stations seem to have been predominantly white, or a similar neutral shade like light beige or gray.

Inside Space Station Mir Inside the ISS

From first principles, what are the color palettes that my protagonist might see if she encounters an alien spaceship and enters it?

Let us assume that the interiors of alien spaceships are similarly alien "white", for similar reasons as human spaceships are human white.

The definition of "white" would be different for aliens depending on the evolution of their visual biology, driven by the spectral class of their home star.

What color might humans see this alien "white" as?

I'm trying to work backwards from an interesting color to a possible type of alien biology, star or civilization. To my mind a potential answer would be something like, for aliens from systems with X type stars and N type of other relevant biological or other characteristic, their white would be our purple, while for aliens from Y type stars and M type of other biological characteristic, their white would be in our far infrared and transparent to us.

I'm trying to find what values X/ Y or M/ N could take such that aliens' white is an interesting (or weird) color to humans.

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    $\begingroup$ This looks like a brainstorming question. Without knowing anything about your alien's vision, the answer might be almost anything. At the very least include some characteristics of their star, so educated guesses can be made about their vision. It may also be useful to know if they are diurnal, nocturnal or crepuscular, and something about the day/night cycle of their planet, as those come into play when vision evolves. $\endgroup$
    – user95279
    Commented Sep 11, 2022 at 9:24
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    $\begingroup$ @Pelinore My understanding is that human white is not a color but the composite of all other colors in the visible wavelength for humans. Assuming that all spacefarers color their interiors in their own visible white for similar reasons as humans do, you can say I'm asking in a roundabout way what might be the visible wavelength for spacefaring aliens. But my question is in the context of a human explorer entering a Rama-like craft and describing it visually. $\endgroup$
    – Pranab
    Commented Sep 11, 2022 at 9:34
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    $\begingroup$ One reason human space stations have white interiors is to make dirt and leaked fluids as obvious as possible. $\endgroup$
    – John Dallman
    Commented Sep 11, 2022 at 10:19
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    $\begingroup$ Unfortunately, this is not how color vision works. First, what you see as mostly white in the pictures I see as mostly light beige (in the first picture) and mostly cream (in the second picture). Second, consider for example our close relatives the dogs; what is white or a neutral color to a dog could be to our eyes white, gray, light red, light green, light purple, or light turquoise--yet dogs live under the same sun as we do. Or consider our more distant but still close relatives the birds; what appears white to us most likely does not appear white to them, because they are tetrachromats... $\endgroup$
    – AlexP
    Commented Sep 11, 2022 at 10:26
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    $\begingroup$ This question is opinion-based because first we don't know the star they originate from. Worse, because we don't know your alien's tech, so physical needs for color like heat dissipation can't be reliably used to answer. Worst, You don't give any description of your alien : if they rely on other senses -like dogs are color-blind relative to humans- or are seeing more than us -like insects with ultraviolets- answers will vary. They could be even be blind, in which case there could be no painting outside "practical" ones! In other words, sun is far from being a main factor for color painting. $\endgroup$
    – Tortliena - inactive
    Commented Sep 11, 2022 at 11:53

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Excluding the possibility of the alien visible light range not even overlapping with human vision as being unlikely to evolve with a star's black body spectrum as the source of illumination...most likely we would see it as white.

Human visual perception involves multiple adaptive mechanisms that attempt to reduce variation in perceived appearance of objects by accounting for differences in lighting. In this case, you're dealing with chromatic adaptation or color constancy: our vision system will adjust to some degree to compensate for lighting with a color tint, so objects appear similar whether they are illuminated by direct noon sunlight, reddened morning or evening sunlight, sky-scattered indirect light in shade, a kerosene lantern at night, etc.

So, if the alien illumination system produces a reasonably accurate imitation of a sun's black body spectrum, we'll probably adapt to it as "white" with little trouble. Possibly their illumination will cut off within our visual range, further tinting the light, but again we can probably largely compensate for the effect, though we might have an impression that the lighting gives poor color rendition. A human might notice the difference on first encountering the lighting, or they might not notice at all until they return to "normal" lighting.

The worst case would be if the alien species lacks (or doesn't value) color vision and chooses monochromatic light based entirely on luminous efficacy to their eyes. Even humans do this: low pressure sodium lights are quite efficient, but mostly produce light in a narrow band that we perceive as yellow. Human vision can somewhat compensate, but there just isn't enough light in other parts of the spectrum for normal color perception, with strongly colored objects often appearing almost black.

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    $\begingroup$ "If the alien illumination system produces a reasonably accurate imitation of a sun's black body spectrum" . . . this means that the oh-so-far-advanced aliens use low-tech inefficient incandescent lamps. The fluorescent and LED lamps which are dominant around us do not produce anything even remotely resembling the spectrum of sun light; instead, they are optimized to produce the same color sensations in the minds of humans, completely disregarding the feelings of insects and birds. $\endgroup$
    – AlexP
    Commented Sep 11, 2022 at 21:22
  • $\begingroup$ @AlexP common fluorescents and RGB LED lights are particularly bad, but with the right phosphors white LEDs can get close. $\endgroup$
    – Christopher James Huff
    Commented Sep 11, 2022 at 22:09
  • $\begingroup$ Is it possible that the aliens' home star itself produces monochromatic or narrow-band light? $\endgroup$
    – Pranab
    Commented Sep 12, 2022 at 12:53
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    $\begingroup$ @Pranab Not really. It might have unusually strong spectral lines, either emission or absorption, but the majority of the emissions are going to be a black body spectrum, varying from blueish white at the hot end to reddish white at the cold end. The coolest red dwarf stars are about the same color as standard incandescent lamps. $\endgroup$
    – Christopher James Huff
    Commented Sep 12, 2022 at 17:40
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    $\begingroup$ I've seen groups of fluorescent lights where some were continuous spectrum while others had distinct bands; entirely interchangeable for us, potentially completely different effect for aliens $\endgroup$
    – Jiří Baum
    Commented Sep 13, 2022 at 12:59
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Human spaceships are mostly white because that spreads light well, and dissipates heat well.

Energy is expensive, light bulbs are expensive and it's cheaper to paint it white than make more lights.

In addition, heat build up is a major issue, so you want stuff to bounce that heat away to keep everything at a good temperature.

Aliens would also use white, because they have the same laws of physics as us. Their white would be our white, because you want all light to reflect.

You colour in things that you want people to see.

Handrails and key equipment is often coloured in something that is easy to see. The aliens could colour this blue or red, depending on whether they used infrared vision or ultraviolet vision. If you turn off the lights and shine a UV torch around you might find all sorts of hidden messages.

Have two species, and have the messages cross to make purple.

If you have two species, one seeing in infrared, one in ultraviolet, the two could symbiotically form purple messages.

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  • $\begingroup$ "If you have two species, one speaking in infrared, one in ultraviolet, the two could symbiotically form purple messages.": er, what? $\endgroup$
    – Christopher James Huff
    Commented Sep 11, 2022 at 18:46
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    $\begingroup$ Blue plus red= purple. Op wanted weird colours. Infrared stuff tends to look redish, since that's closest, ultraviolet looks blueish. $\endgroup$
    – Nepene Nep
    Commented Sep 11, 2022 at 18:47
  • $\begingroup$ For humans, the spectral color violet looks purple because stimulates the cone cells similarly to a mix of blue and red light. Ultraviolet and infrared don't stimulate the cone cells at all. $\endgroup$
    – Christopher James Huff
    Commented Sep 11, 2022 at 18:53
  • $\begingroup$ Light is a spectrum. Something that is ultraviolet reflecting may also be a bit blue, and something that is infrared reflecting may be a bit red. It doesn't have to be, but op wants purple spaceships. $\endgroup$
    – Nepene Nep
    Commented Sep 11, 2022 at 18:54
  • $\begingroup$ Might be but isn't necessarily so, any more than something being red meaning it is probably a bit green too...the whole reason color vision is useful is that reflectivity can depend strongly on wavelength. Look at some infrared or ultraviolet photographs to see how drastically different things can appear in those wavelengths. $\endgroup$
    – Christopher James Huff
    Commented Sep 11, 2022 at 19:06
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What humans perceive as "white" is an equal reflection of the wavelengths the photoreceptors in our eyes are sensitive to. Which are the wavelengths we interpret as "red", "green" and "blue".

Aliens who evolved around a different star and in a different ecosystem might have evolved photoreceptors which are sensitive to other frequencies. For example, if the wavelengths their eyes are sensitive to are of lower frequency than ours, then they might not be able to perceive what humans perceive as "green" or "blue", but instead perceive "red" and various colors humans would classify as "infrared". So the lighting in an environment for these aliens would probably appear to humans as dark reddish. Similar, if the aliens were sensitive to higher frequencies, then their preferred lighting would appear blueish to humans. It might even be possible that their visible light perception is completely outside the human range of perception. So their environments would look completely dark to a human, while it would appear brightly lit to them.

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