I'm not sure I understand 'digital' painting. Is it not just creating a layer over your original image and then just fiddling and using it like its tracing paper? It looks effective but I can't help feel it looks good because its so easy to manipulate? After all, you've got a tool to match the colours. All you need to do is select a brush, drop the opacity of your 'digital' layer and trace away?
edit;
this post just sounds rude. I've looked over some more of the digital paintings in this thread and can see it involves some skill, but why not try the real thing?