MyPaint 'QuikUI' idea:

Here's a possible design for a minimal UI for quick change of some brush parameters.



Tabs are
- Size (1 pixel to 38 pixel diameter in this example)
- Luma / Brightness
- Chroma / Palette (user-selectable?)
- Bottom tab / extra tabs user-configurable? (Opacity, speed etc)

It would be quite quick to tap on a tab and then pull stylus to the desired setting (little movement required).