Probably I was a bit confusing about the on-canvas functionality: that's an idea for the future. For now I only propose the color adjustment / gesture widget.
http://www.vmc.org.pl/pliki/gestures-before.png http://www.vmc.org.pl/pliki/gestures-after.png
I hope it explains my idea a bit. The second image shows what would happen when pressing LMB on the icon with the two-way arrow. Pressing Ctrl or Shift would change the little icon to show a different gradient. Depending on the currently active color, the horizontal bar would have to pop up in a slightly different place so that the current color is under the cursor, except tf there's no space because the icon is to close to the edge of the screen - in that case the bar would touch the edge.
Regards, Krzysztof Kosiński.