How to?

That might be more difficult than you expect, especially the length.

How easily one can change the colour depends on the image, the actual and the intended colours.

Changing black hair to blonde for example may proof difficult if there is not enough information in the image.

Could you post (part of or a lores of) the image?

Rather than change your own hair, it would be easier to composite hair from other photographs onto images of yourself.   I would not be in the least bit surprised if some salons have software that will do this for you, and I bet there is an iPad app or two that will do much the same thing.

There you go:  There are loads of them.

http://itunes.apple.com/us/app/change-my-hair-free/id392928307?mt=8

http://itunes.apple.com/us/app/hairstyle-magic-mirror-change/id429947895?mt=8

http://www.youtube.com/watch?v=wn3OtYBdlvc