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Green also could be a chlorine
Pink hue is made by methane absorption lines in spectra, it is a prediction of color of a T-dwarfs and warm gas giants.
Yellow/red is hydrocarbons/dust, as like on Titan and Mars.
Athmospheric chemistry is a hugely complicated mess, because adding to every compound that can be a part of it either in gaseous form or in particulate form, there's also layering. Chlorine gas, for instance, is significantly heavier than oxygen or nitrogen and also heavier than CO2
, which means that unless it's pure (and it's very unlikely to be pure because the elemental abundances favor lighter elements), it should concentrate at the bottom of the atmosphere, probably generating a thickly hazy and greenish layer, although this would probably be further changed with the impact of different temperatures, leading to selective condensation or evaporation. It's all pretty complicated, it's all subject to active scientific research, and therefore I guess SE can't dive deeply into it.
However, I do think SE needs to do a better job explaining the atmospheric colours it shows. Info about hazes or suspended particles needs to be added to the basic atmospheric composition info. Otherwise I'm afraid the program would be contributing to the misconception mentioned in this thread.
And perhaps show more blue skies than it does now, making other colours rarer. After all, we live in a solar system where 4 of the 9 worlds with visible atmospheres have blue skies and all the others have them down to a certain depth, where hazes and dust begins to take over. Blue should be more prevalent in the Space Engine universe, methinks.