This was, surprisingly enough, quite soothing. I ended up just staring at the screen as I drew.
However, I do feel it's a bit too simple. The game doesn't really have much variation to it. Apart from the music, it's fairly the same throughout, apart from switching of colors or speed and such.
In the shape of the brush, I feel you could've added something. Another couple of variations of it. Spirals, stars, I don't know, but as is it's far too simple to reach high up on the scores.
Maybe make it easier for someone to pick colors. I don't know anything about HEX, so I just ended up inputing random numbers. Still looked awesome, byt all means, but maybe having a sort of diagram or something, or sliders instead.
Also noticed that when I set the speed to below 1, the cursor would shoot off screen and away from my cursor. Guess that's a bug?