You know, I just had a crazy idea.
Okay, so Rannoch's sun is more orange than Sol. That means it emits more visible light in the red and yellow part of the spectrum than Sol.
Now, as you may or may not know, the colors we see (unless it's the actual light source you're looking at, of course) is actually reflections of wavelengths that were not absorbed by the material. A blue ball for example, is absorbing all the wavelengths of visible light, except blue, which gets reflected out. Thus it looks blue.
If Rannoch's sun emits slightly orange light, then it could mean that Quarian skin might choose to absorb those particular wavelengths of light, which would of course means it reflects back the rest of the wavelengths. That would conclude that the color of their skin would actually be a "cool" color, ranging from light blue to purple
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You know, reading this over before I post it, I almost want to just say "ignore this". Since the entire theory is based on the assumption that Quarian skin will want to absorb more light, which it may or may not.
Basically, it was just a thought. Move along people.