I paint with acrylics, which dry fast. That's why I cannot keep the paint on my palette forever. Whenever I mix a color I like, I record it as a formula in my color diary, and that's how I can re-create it again with accuracy. 

It is a tool for convenient mixing three or four colors into a grid with proportional gradations.

I use it to test my paints. I can mix two colors with white or black, three or four colors with each other, or a color with two different tinting colors. 

It's a printable implementation of Bruce MacEvoy's paint wheel. 

I often paint with a limited palette; with this tool, I can visualize the color gamut of my paint selection.  

Here, you can download Bruce MacEvoy's paint lightfastness testing matrix. I do the test occasionally to remove weak pigments from my palette. 

You can read about the testing methodology on his website.

If you're doing watercolor or any other water-based paint test - prefer thicker paper. Ensure that your printer can handle the thickness, though.