Help, please?

Hi! First, I'd like to say how much this community has helped me improve my icon-making skills. But there's one thing that I desperately want to do and I know it must be rather easy but I haven't learned how to do it.

Say you convert a color image to black and white but you want to add color to the subject's eyes or hair. How would I go about this?

I use PS 7. Any help is very much appreciated. :)