Selective coloring is a quite easy but interesting concept. First off you start by finding a good image to perform your magic on, I browsed flickr and found this image. (Sorry for using this if you own it :)
We start of by opening our image in the pixlr editor and duplicate the background layer.
The background layer is by default selected so just click “Layer → Duplicate layer” in the menu. You now will have a copy of the background in the right hand layers box.
We now turn the layer copy into grey scale, to achieve this you click “Adjustments → Desaturate” in the menu.
Now starts the real work, with the Eraser tool selected you start to erase the parts of the layer you want to be color, when erasing the grey layer the color background will shine trough.
A tip is to work with a larger brush in the beginning and choose a smaller one when doing the edges of the color area.
Here is a other picture done with the same technique.