This display shows the mixture of the color bars below and right. You can add any amount of the color to the left by clicking on its bar. You can switch colors by clicking on any of the other two color bars.
Clicking on the color display will show the values below.
If you click on the value display, you will get a different display mode, where the color to the left is added automatically such that the brightness is about equal in the lower right triangle. Just try it. Clicking again gets you back to the original display.
Here is the source.