I really like it.
I could only sketch with grey. I didn't log in, and I didn't look around for instructions. Eventually I gave up and looked for instructions, but there was a big wall of text. So, I know that cursor keys do size of brush and colour (but colour is asymetric).
You might want to consider a paint style toolbar. For people like me. Just something really very simple - brush size and brush colour.
This is my first impression after about 45 seconds of play. You may not want users like me, (people who don't bother to read instructions) and if that's the case then that's fine!! It's a good thing to cater to your market.
