This is still a beginners guide for the simple fact that I'm a beginner, but thanks to some very good plugins it's a breeze (even for me).
Step 1: Things you need
Paint.Net of course
For the plugins I've downloaded 2 packs
Ed Harvey Effects
(or download below)
These give you lots of effects to play with ;-)
We're going to use, besides the standard tools available to us,
Color Tint (Ed Harvey Effects)
Extract Channel (Ed Harvey Effects)
Color Balance (BoltBait's Pack), this one's under the Adjustments menu instead of the Effects menu.
And most important, a photo you need or want to edit.
No I didn't fly all the way to China to take a nice photo. I found this photo on openphoto.net, it's free to use under the Creative Commons Attribution-ShareAlike license, so perfect for this instructable.
Step 2: Lets get started
It basically comes down to getting a selection around the object you want to change and then changing that selection only.
One possibility is to duplicate the original to a different layer.
Erase the bits you don't need (use the Selection tool to erase larger parts)
Apply an effect, I played with the Color Balance here, to the remaining bit and flatten the image
As you can see this is a painstakingly slow process, but I guess for small images it would get the job done.
Also, if there's more then 1 color, then it gets even more difficult.
It works, although the edges are not great and the letters turned out orange. But that's because I didn't erase the letters and didn't zoom in close enough to erase closer to the edges.
Like I said this is a pain in the *ss to get it right.