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.
Step 3: There's Got to Be an Easier Way
Duplicate the Background layer and cut out a selection like in step 2.
Use the Sharpen tool (Effects-->Photo-->Sharpen...) to get a beter contrast between the letters and the jacket. I used the maximum value, but maybe for other color it could be less.
Duplicate this layer a couple of times. One will be used to isolate the letters, another for the jacket, maybe one for backup, just in case.
Zoom in on the letters for easier working. Use the Magic Wand tool with Selection Mode: Global and select a red part. Invert the selection and delete.
There's still a little blue edge, so repeat a global selections on a blue bit and delete to get a nice cut.
On to the jacket.
Step 4: Cut the Jacket
The CMYK-Cyan channel gave a good result. Use the Magic Wand tool to select the jacket, there will be some extra selected, but it'll be minimal and easy to erase with the Eraser tool.
Use the Extract Channel tool and the Magic Wand tool to get an as close cut as possible. This takes a little practice, but it ain't rocket sience ;-)
Step 5: Lets Put Things Together
With the jacket layer selected, use the Magic Wand tool in Flood Mode: Contiguous and select outside the jacket.
Then select an original and delete the selection.
Next select the Letters layer, again with the Magic Wand select outside the letters, but this time in Flood Mode: Global.
Invert the selection, select the original layer again and delete.
You should end up with a layer that just has the jacket with no letters.
Now using the Color Tint tool or the Color Balance tool.
To get the jacket white I needed to combine the two effects. First Color Balance as close to yellew as you can. Then with Color Tint go to white.
Delete the layers you don't need and you're done.
If you find that some edges are too sharp, use the Gaussian Blur effect to soften the edges a bit.
Ok, now it's up to you, practice and play with these effects.
Hope you enjoyed it, have nice day and don't forget to vote ;-)