Because I have been promising to do it for months, and I finally got around to taking some screenshots while explaining it to someone, here it is. It works in either PaintShopPro or Photoshop, and I can't imagine why you couldn't do it in any image editor that supports layer masks, because that's pretty much all you need. It's very easy, once you get the hang of it, and imo, produces more lifelike, realistic results than colouring each object separately does.
more >