Shego and her family were exposed to a multicolored space meteor, which gave them each their unique abilities and varied skin tones. Which means, since Shego's powers were radiation-based, couldn't Kim have simply defeated her by placing her behind some lead plates, a pane of leaded glass, or simply tossing lead powder in her face?
The Gross sisters were blue because their skin was dry and ashy. What, they never showered, or at least went swimming or something? Maybe Drakken just needs a good moisturizer.
If you think that's weird, have you ever seen all the green-skinned villains in the 1969
Spider-Man series?
What, was there gamma radiation in NYC's drinking water, or something?