When plein air painting or painting in the studio and I am struggling with, "what is that color?" I use a 3" X 4" Index card that i have used my hole punch on.
AFter I mix some paint that I think might work, I put a small amount of paint next to one of the wholes and is using a photo I put the whole over the color I want.
And then I can adjust the color and or value. This eliminates what I "think" the color should be!
Here is another example: