Seeing past color to read value is an essential painting skill. It can seem impossible when you first start to practice it, but a couple of questions can help bring the task into focus.
"How dark is the part I'm about to paint compared to what I already know?
Value is relative. Every area of a painting is assigned a value compared to its adjacent shapes. Look for something darker and something lighter than the shape in question to discover the range within which it falls.
What is the darkest thing in the picture?
What is the lightest?
If you are unsure of the value of a given shape, ask yourself if it is closer to the lightest thing or the darkest. That gets it in the ballpark. From there you can bump it a little in one direction or the other till it feels just right.
This week, make a 5 value ( White, light gray, middle, dark gray, black) monochrome study of an image that appeals to you. To assess the information that is available from the study, ask where you need more subtlety or more specificity. When you are sure you have answered the big value questions, use the study as a road map to guide a full color version of the subject.
Bring the study and the painting to the critique.