Such a speed up, but I didn't realize it was this easy.
Just Alt click on any spot in the application, and your selected color becomes the color below your cursor. So, when you've mixed some paint and like that newly created color, just a simple Alt click sets the color. And this will work on any reference image that you have loaded as well.
If you liked having a board to mix your colors with natural media, here are two things with the program. While it would be nice if we could have more than one painting loaded in Artrage at the same time, with one of them being your palette, Artrage keeps the interface is a little easier to use by having only one painting. So here's an approach which works well with the Alt Click.
Make a painting which is nothing more than your palette. Then mix the colors around with the knife. Save this painting and load it as a reference.
If you are painting from a photograph, start with the photo loaded into a layer and select your fundamental colors right from the photo as a starting point.
After loading your "custom palette" reference photo - You can Alt click on any spot to select a color. Zoom in first if you like. Or load it as a layer. You can move it off to the side, or take some colors and put it on your main canvas. Easy enough to paint over it or delete when you're done.
Quick and easy, saves time hunting down and remixing that color. Helps your painting stay consistent.