How strong is your painting. You think it looks great, or you are having problems with it? Either way, the test is how it stands up as black and white.
Use any of these tools, and you will probably find your answer: Notan, value studies in charcoal or black markers, iphone's Value Viewer app, Gray Scale & Value Finder by The Color Wheel Company, Edgmon Art's red plastic viewer.
Contributors to the blog of Oil Painters of America write really good articles, whatever medium you use. Rich Nelson writes The Value of Value, with this little excerpt:
Tap the link to read the full article and see examples. Worth the time.
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