If you watch any episode of Buffy with Spike in it all the time, keep an eye out for this:

Spike's hair is obviously dyed, right? That is no question. But, if you look carefully, you will see that in one scene, he has a little bit of brown roots, and in the next, NONE! Over and over, his hair isn't perfectly dyed, then it is, then it isn't again!! They should be able to keep on top of that, wouldn't you think?