TL;DR: It's part of class identity it has.
Not saying it wouldn't be nice to colour skills or that colours are a necessary aspect of class identity (arguably for most classes it isn't) but I'd say it's just as part of green disease/(yellow kitten for Scourge) aspect of Necro or azure fields of the Guardian, various palettes of Revenant for different legends.
Most people either played long enough to stop caring, learned to love it, or care more about solutions for the class' awkward drawbacks in gameplay or spec strengths.
