I was doing some tests with bidirectional painting — I just mixed colors from Brush and Canvas. I did this video,so that you can see results -> http://lukast.mediablog.sk/videos/nice-compositing-krita.ogv
It nicely mix colors. When you select yellow color and you will paint over the painted pixels you get the next closer color (first I selected yellow hue which was closer to red, then the yellow closer to green — see results in short clip (2,3 Mb))
Actually it is in svn but you have to turn it on in source code using instructions that I wrote to commit message.
Don’t forget to turn on “Paint Incrementaly” too.