I forgot to mention it is a photo referenced study, I can't accept any praise on it
@Taron: Also forgot to mention I've missed some color dynamics, basic HSV "jitter?" dial/s would be a handy time saver!
Since the GUI is already a bit crowded, I would suggest the crazy idea of using the RGB dials:
mouse hover + shift over RGB makes the dials become HSV jitter dials. (maybe with some visual feedback like different color for the dials... )
Well, maybe it would be be a bit missguiding it being jitter dials rather than just changing the color based on HSV...
maybe Shift could be used to change the colors and Alt could be used for the Jitter...
Since there is no brush spacing/sampling I guess at least the jittering could work on each new stroke...?
Well, maybe none of this is possible, but I had to mention it! sorry for the long brick!