One way of checking your values in a painting is to put it in photoshop and desaturate it. It also helps with a composition you are uncertain about. Without the hue distracting you, you can see the structure (or lack of structure) in the painting and make necessary corrections. This painting is in progress. It is a sunset scene across the wide Mono Valley on the east side of the Sierra taken from high up on the White Mountains with( unfortunately) what I call the "Jesus Light" reflecting into the camera. I needed to test the values because I knew my brain was lying to me.

Another way of testing if you don't have photoshop is a piece of transparent red plastic. All color looks the same and you see the values.

SL Sorensen