Make sure to read this as well. It's a good read, although I personally think balance and convenience should have (slightly) higher precedence over minimalism.

Are you left handed? I was wondering because all the panels are docked on the left side of the canvas. I'm right handed and it feels more comfortable having my UI to the right.

Also, I won't beat around the bush. The 4 pen colors thing is a Bad idea. With a capital b.