Thanks be to Flikr users like Courtney Rhodes who put their photos up under Creative Commons licenses and allow digital artists like myself to use them to learn on.  To the left is the original photo she posted on Flikr.  Below are eight retouched renditions, each with a different hair color: Bottled Black, Natural Black, dark brown, light brown, auburn, ginger, strawberry blonde, and blonde.

Ginger Hair Photoshop
Light Brown Hair Photoshop
Black Hair Photoshop
Strawberry Blonde Photoshop
Auburn Hair Photoshop
Dark Brown Hair Photoshop
Glamour Black Photoshop