Whether or not you are a purist, there are times when you have to change the color in post production of something you've photographed. This video will tell you how you can easily alter hair color of a portrait to whatever hue you want.
Colin Smith from PhotoshopCAFE published this nice quick tutorial where he covers a lot of ground in just three minutes. You will learn about making quick selections, refining a selection, masking, working faster using shortcuts, and of course, changing the color of the hair. Smith is not only showing how to achieve uniform nuances, but also how make hair multi-colored and funky. The tutorial shows how you can alter the lightness and contrast of any isolated elements. Turning the original light brown hair into blond is a little tricky, as this change is not so much in the color hue, but the intensity of it. Smith will show you how to do all those alterations so they look as natural as possible.
Using that knowledge you can easily transfer it to other elements you want to change color of, like eyes, clothes, objects in the environment, you name it. The principle is the same as long as you master the technique of masking elements and use adjustment layers that are constrained to a masked area.