The traditional way to do this is to have another bitmap, the same shape as the one the user sees, but with different coloured regions on it. When the user clicks, you hit test the hidden bitmap and pick up the colour.

Interesting idea. I'll have to see if that works in VB. Sounds like it would work.

Oh, and can you make it alpha-transparent when it's not the active window ?

I don't understand the question. Are you saying that the Region Window (non-rectangular window) feature isn't working when the window loses focus? I'm not getting that. The windows have rounded corners regardless of whether or not they are the active window on all the systems I tested on.