This is actually kind of fun. Tedious with a mouse, but fun none the less. The magic paint brush lets you pic up color from the underlying layer whilst painting away. Blending and all that other stuff is done with adjustable presets allowing some of the underlying to show through, if you want.
In playing around with this tool I've discovered that the lower res, smaller files are easiest to work on and get acceptable results. I tried working on an 8 megapixel image and nearly lost my mind sampling small areas at a time and trying to paint. It is much easier to reduce the size of the image and dpi and work from there. The compression of detail doesn't matter, actually helps in putting together an impressionistic image...like Painted Jalama Dunes you see here.
I really need to invest in a pen and tablet if I am to explore this option further. Of course to get there I need to sell A LOT of prints.