Above is an example of a Grisaille (Gray monotone painting) done directly from life. By mixing Raw Umber with White to create a range of value to render light on the form giving the illusion of three dimensions.
Color is not needed to depict form, space or to tell a story. Just look at the history of black and white film photography. It is said, "Value does all of the work but color gets all of the credit."
If you're having a tough time mixing and matching a color make sure you're in the correct value range first. That said, as long as the value is correct you can "push" color to the point that it is not representative of the actual thing that you're painting but will still read correctly in the context of the painting as a whole opening up a world of possibility for the artist.
