Really nice program, controls are a little bit funky
For the airbrush
You may want to consider using a gaussian distribution random, I implemented it instead of yours, it just feels a little smoother used as an airbrush
(it's the random that forms a bell curve, so points further away are a lot rarer, it makes a smooth gradient circle when you hold down the airbrush)
The linear-random skewed shape thing you used is pretty realistic too.
I think it'd be cool to build a whole drawing program purely with different styles of airbrushes, that emphasizes shading. And maybe automatically apply the blurring from the smear so that the shading is less noisy.
... GPSET TX+RNDG()*THICK,TY+RNDG()*THICK,DRAWCOLOR ... 'GAUSSIAN DISTRIBUTION FROM -1 TO 1 DEF RNDG() RETURN (RNDF()+RNDF()+RNDF())/3*2-1 ENDJust a thought. Cool program! 👍