In this great tutorial, Aaron Nace from Phlearn teaches you how to change hair color in Photoshop. He shows the trick to changing red hair to brown, black and blonde and this video also gives great insight into and cause for practicing your skills with blending modes as well as selective color and levels adjustment layers.
Why stop at hair? Take some of these awesome little tips to help change color anywhere in your own photographs.
[via Phlearn]
Some nifty info here. I would use more precision in the masking tho, instead on relying on painting color back in where it may be missing and whatnot. I know he was being quick and dirty there. Sometimes a channel layer can be use as a luminosity map for going blonde.
I ended up trying to convert darker hair and the selective color bit didn't quite work as well. I ended up masking like Spy Black mentions. I got some moderately convincing results, but it's certainly not a technique I'd want to have to lean on all of the time.