Actually getting on topic now. Here's a tutorial for all your recoloring needs.
I'll be using the GIMP program for this. You can download it at: http://www.gimp.org/downloads/
This is the image we'll be playing with today.
The first step is to obviously open the image file with GIMP. Then we pick the free selection tool!
Mark the area you want recolored. In this case I'll be recoloring a part of the girl's hair.
After that you click the colors tab and pick the colorize option.
At that point you just mess around with the options you're given until you get the color you want. You can save your presets by clicking the + (plus) symbol.
And that's all. Simple right? Unfortunately, the same can't be done for recoloring stuff to white (unless you want it to look really ugly...)
But there's a way around that!
First we select the area we want to recolor, then we pick the airbrush tool. We set the mode to 'Color' in our tool options window. Of course you should make sure that white is your primary color before doing anything.
You click over the area and then it should look gray at first. Like this:
Then you grab the Dodge / Burn tool and set the type to Dodge. Your range should be Highlights in most cases.
You click, click and click until you get the white that you want.
Voila! And that's all folks. Hope this helped with all your recoloring issues.