I would not say that a painting can't be successful if color if it is not successful in black and white, but it does seem like a good tool for looking at your work in progress. Seeing your work minus the strong language of color allows you to see things you won't see otherwise. This is similar to seeing your work from a distance, through a camera lens, or on a computer screen. It gives you a different view, which can be revealing.
I just did a few grayscale versions of some 8"x8" print/collages to see how they looked: