“Mono” means one; “chromatic” means color. A monochromatic color scheme, then, is one in which only one color is used. Just as using a gray scale can help you understand “value” when drawing, using only one color in a picture can help you understand “value” when painting. Darker colors are good for this type of painting, as you can create strong darks as well as lighter tints. Try colors like Burnt Umber or Ultramarine Blue.
Another benefit of creating a painting in one color is that it enables you to see how lighter values recede and darker values come forward. For example, if you look at a picture with mountains in the background, you’ll notice that the further away the mountains are, the lighter in value they appear to be. Closer objects in the picture, such as a tree, fence, or building, take on much darker values.