Currently in P2020 we can load some brushes with multiple colors via the color mixer panel which is very useful. But it is a bit blind and awkward and unknown until we start painting. And we have to setup a color mix (to select from) to get a desired result.
Photoshop CC has a much better solution whereby you can select the colors from any open image, and the brush shape selects the range of colors under the brush shape. While one could still make a color mix, it is far easier to sample from a set of pre-made images or from an open reference image.
See this video to see a demo:
So my suggestion for Corel, is in a future version of Painter, to take some cues from this video and dramatically improve color mixing selection by allowing sources such as an image instead of the color mixer (although we probably can sample from an image in the color mixer; why not sample from any open image).
Secondly, to be able to see a reference sample image of the colors being sampled...like a ghost image of the brush shape filled with the newly selected colors.
And thirdly have an option for the color sample to be the same size and shape of the brush being used to use multiple colors instead of a simple circle with radius control (and blindly not seeing what is being selected). A "selection" (marching ants) overlay of the brush in use, over the color mix image, would clearly show the user just what colors are being selected before actually clicking to commit to the selection.
Thanks, looking forward to a much better multiple color selection GUI in Painter.
^ All of this. Big YES.