Or Andorians just don't give a crap about the differences in human ethnic groups.
I mean why would they we're all a bunch of damned dirty aliens to them.
And, that would have made for some interesting (and challenging) storytelling...
It would be interesting seeing a 'dark-skinned' human - brownskin, dark-brownskin, or yellowskin - calling out Shran or a likeminded Andorian on their racist BS.
Ok... please don't label me a scriptwriters' apologist, but...
Do they only see "white people" or do they only see "pink skins"?
This is pure speculation, but, what if... The Andorians see in a
slightly different spectrum of light than humans? I mean, don't we have animals on this planet that see things slightly differently based on their environmental conditions and survival needs?
They inhabit an ice cold moon, and live deep in underground caves. The surface temperature of said moon rarely gets above freezing, and some of their caves and tunnels are actually excavations and hollows within the moon's surface pack ice itself. They seem quite adept to this chilly environment and it might have affected their biology over time. Their body temperatures are almost certainly lower than that of humans. And with regularly being out of direct sunlight, perhaps they developed retinas which react more to slightly shorter wavelengths of the EM spectrum, especially areas of the infrared which we simply cannot see. visualizing warmth in exceptionally cold environments would definitely be the advantage on their world (especially with those hot little worm things dropping out of a ceiling of ice without a moment's notice).
And lets not forget, an entire subspecies of the Andorian race (the Aenar)... is actually
blind. I mean, with that alone, we've evidence of some interestingly askew and genetically based dynamics of the ocular going on.
So, even though there is no "canon" proof of this phenomenon (and I care not), I would postulate for speculation's sake:
Perhaps due to a minor variation in visual perception, the Andorians actually see ALL humans as "pink-skins", simply because our average body temperature tends to be far warmer than theirs,
regardless of the epidermal pigmentation present in any particular specimen. Maybe even Vulcans are frequently referred to as a different (albeit, derogatorily implied) "<insert hue>-skin" since they are typically even warmer than humans? Of course, they've interacted with the Vulcan's for far longer, and they tend to say "Vulcan" with enough venom already, that it's hardly necessary to highlight their xenophobia even further with that.
"Pink-Skin" might have been Shran's single greatest contribution to Federation culture, since it seems he coined the term, but I would argue... let the Andorians be the bigots they were allegorically written and characterized to be, rather than dropping this on the door step of the writers as a blunder.