Marked vs unmarked! I vaguely gestured in that direction but shall edit in a specific mention.

I noticed the problem with darker characters (specifically, dark furred anthropomorphic cats, and doesn't that speak volumes) years ago, it was only when I was stuck with a particular character and picture where I couldn't cheat that I decided I needed to make a concerted long term effort to fix the problem. Clearly YOU should start drawing Friday Night Lights fanart :)