I watched the above mentioned video, and this seems to provide the solution regarding the “drawing black” issue. What basically happens is, that instead of drawing the final color into the rendertarget, just draw a single color and then lerp between white and a draw color in the whiteboard material so that the “additive” stuff never meets black.
I did a quick and dirty test, extending this to be able to select colors, doing it randomly in my BP for testing:
In the brush material, have a color as you did, this is now just a Color Selector, so a basically a mask gets painted into the render target.
In the whiteboard material, the rendertarget is just scanned and colors are lerped in.
In the whiteboard blueprint, here’s just a simple random selection done, quick and dirty.
The result
Just as some option to test with. The number of colors is limited (could maybe extended by using 2 rendertargets), also the order of colors is fixed, last in chain wins. But maybe it is helpful.