Another useful tool is the value finder. It is a square of mid value grey with a hole in the middle. Hold it over your work over different colors. Is the color lighter or darker than the grey? The white value finder works the same but can be used for seeing and understanding lighter values.
Have you tried either of these tools? Do have trouble sorting out the value of colors in your work? If so, try one of these to see if it helps. Leave me a comment and let me know what tools you use to determine values.
Value scale and finder from toadhollowstudio.com