Picking skin/fur/scale/feather colors in the light of a white sun (F-star)

Question

As I have been developing my world, namely a massive moon (0.9 Earth masses) oribiting a gas giant (about 3 Jupiter masses), I have confronted multiple times the question of what colors should the life forms on the planet feature.

For the plantlie I have used a calculator that helped me find out the best coloration based on the star's mass and age, the result's been that the plants of the planet could come in two sets of colors: Violet (to indigo to blue), Chartreuse (to lime to parakeet) and maybe also something in between the two like combinations. For in depth explanation the reason for the former would be to reflect the most harmful radiation while in the case of the latter it is to absorb it for additional energy.

Unfortunately I haven't been able to find an equivalent to determine skin/fur/scales color, so I was wondering if you could help me with that. I have considered following the same guideline as the plants but I am not sure about it, I mean, real world wise it wouldn't make sense given that our skin isn't green like the plants right?

So far what I have concentrated the most on the humanoid dominant species of the planet, giving them a skin color comparable with humans but much darker kind of like those of Mexican Ethnicity, Native Americans or Indians. Do you think it would make sense? Or should I consider a different set of colors? Is there also a guideline for fur/scales/feathers?

Additional information:

• Star mass: 1.31 suns (F-star white sun)
• Atmosphere of the planet: Earth like with additional carbon dioxide, noble gases increasing the density and most importantly shielding gases.
• Orbital inclination: 30% degrees leading to seasons more extreme than earth's and light distribution (?)

Answer 1

The colors of animals usually have less to do with sunlight, as they do not photosynthesize (unless you want them to). Human skin tone is affected by exposure to sunlight (tanning), so if the sun in your world is hotter or more intense than on Earth, humans would be darker on average than they are here. Feathers are colored to make species recognizable to each other and attract mates. Some animals, like frogs and some kinds of insects, are colored green to blend into foliage; if the plants in your world are blue, then these animals would simply be blue.

Answer 2

Any photosynthesizing life (most obviously plants) are likely to maximize efficiency by reflecting the peak wavelengths of the star. For example, our sun’s energy output peaks in green light (around 500nm) and our photosynthesizers are mostly green. It's a bit counterintuitive since you'd expect plants to want to maximize energy intake but there's more to be gained from maximizing energy consistency. My point here is that the interaction between a star's radiation and the color of life on an orbiting planet is complex.

In a forest full of violet plants, there’s going to be an obvious evolutionary advantage in being violet. This is just about the only way star color impacts animal color. But you’ll note that not every animal living in a forest on earth is green. That’s where the other factors come in.

Assuming your animals don't photosynthesize, coloration is going to be determined by several factors including environment, how they hunt and/or avoid predators, how eyes work (theirs and their prey/predators), reproduction, and evolutionary history. Most of these factors are up to you to decide. Just, don’t overplay sexual selection; it’s real and Fisherian runaway does happen but it should be used sparingly otherwise your world will seem implausible.

The intensity of the short wavelength light hitting the planet can impact skin coloration if the animal doesn’t have other mechanisms for dealing with it. For most animals on earth this is handled via hair, feathers, or scales. For evolutionary reasons, humans ditched most of our hair and rely on a chemical that changes skin coloration (melanin). This raises a concern about the proposed coloration for the dominant humanoids. The question describes them as having “skin color comparable with humans but much darker” implying that they also rely on something like melanin. That will only work if the entire species lives in one location. Hairless animals living near the equator are likely to develop darker skin while those in polar regions are likely to go lighter.

Always consider evolutionary history. Working out the full details of a world’s evolutionary history isn’t necessary (although it can be pretty cool when it’s done well). It is important to have some basic idea of how things got to where they are in the world’s present. Even if that’s as simple as “my giant marine filter feeders evolved from terrestrial bear-like ancestors just like whales on earth”. That’s not super original but it at least gives you some constraints, so your whales don’t suddenly sprout feathers. This same principle applies when working out coloration: know some of your world’s evolutionary history so you can decide how your animals can and cannot be colored. So, for example, earth mammals mostly use melanin for skin coloration but melanin doesn’t do blue so any blue skinned mammal would have to evolve a secondary mechanism to get there (e.g., structural coloration).