I usually keep a separate layer for some samples and testing a mix (although I seldom "paint" that way)
this but it's unwieldy. As is the color sampler (as another thread recently adressed).

How about a new pod replacing the color sample where you can switch between a list view and a palette
view (and like the color picker this can be an image or a smaller AR painting that can be replaced/shared)?
When you add a color to it, it appears as a color puddle that can be moved around and sampled by alt-klicking
but also mixed in the middle (being able to set this to real or RGB-mixing separately from the painting itself
would be nice but not a must).

Something along those lines perhaps.