Personally, I prefer a colored outline to a black outline, gives the sprite a softer look. Most people don't like them (just like they don't like all black) as they like a mix of it. However, on humans, a mix is typically best :P On pokemon I prefer the all colored.
Rough Showing on "softness" (Hope ya dont mind)
And sorry, it is gonna be somewhat horrible, haha. I did it in like 4 minutes in paint <_< >_> Obviously these aren't the colors you'd be using for the outlines :P