Here are some of the color charts I worked on for the color theory class I'm taking from Gabriel Lipper. The idea is to evaluate colors in terms of their value range (meaning how light or dark the color can be made). For example, yellow is a color that is already pretty light coming out of the tube, while blues and greens inherently have a darker value. Each chart has a value scale from dark to light made with black and white paint, and then each color is matched as closely as possible to the values represented in the black and white row.
On the top are the charts pictured in color as painted; on the bottom, they are shown in black and white, which shows more clearly the values that each square carries.
For this exercise, I didn't have the right size of canvas panels, so I used some old pieces of 300 lb. watercolor paper that had been used previously for various watercolor and ink experiments. I kind of like the effect of the uniform squares over the images!