When I decorate a cake with colored buttercream, after I smooth it, it has different shades in different places, like this:

http://cakecentral.com/g/i/3111614/a/3409545/wall-e-cake/

I was okay with it on this cake because it was outer space, but why does this happen?

That happens to me sometimes, too.  Usually when I smooth with a scraper dipped in warm water (even when wiped dry) or if there is a lot of condensation on the cake before crusting.  I the the moisture is basically removing some of the food color from the frosting.  I've been able to prevent it by using a different smoothing technique and not touching the cake when there is condensation on it.

Great, thanks for the info! That's really interesting.