John Singer Sargent, "A Parisian Beggar Girl"
Tone the canvas before you paint
While the first three secrets I've written about in earlier blog posts work across the board, this — and most of each of the methods to follow — is not something you need to do to make things work. They, however, are great techniques to try, and they will help create color harmony.
John Singer Sargent, "A Street in Venice"
Underpainting the canvas with a solid color will pull together the colors in your painting. What color to choose? Here, John Singer Sargent has used a creamy almost-mid-tone for A Parisian Beggar Girl and a darker ground for A Street in Venice. Notice how he has used scumbling across the surface, allowing the ground color to show through. The creamy underpainting of A Parisian Beggar Girl provides a foil for the subtle lavender brush strokes above it, the eye mixing it into the rough neutral colors of an old wall.
You could also consider something that will contrast with what you will paint on top of it. You might want to choose complementary colors, or warm/cool contrasts. Many landscape painters, because of the predominance of greens in their paintings, choose some kind of red, magenta, or burnt sienna.
Charles W. Hawthorne, "Portrait of Henry Hensche"
Charles Hawthorne, in his Portrait of Henry Hensche, has used a stronger set of yellow–purple complementary colors, with a vivid pinkish-purple underpainting, visible at the bottom of the painting and peeking out from underneath other places, contrasting beautifully with the yellows and greens of Hensche's slicker and the surface of the water.
Charles W. Hawthorne, untitled portrait
In this untitled portrait, Hawthorne used a faintly visible yellow underpainting to contrast with the strong purples that dominate the painting and and pick up and create continuity with its yellow highlights. In each of these paintings, the color of the canvas, toned before painting, creates interest, continuity, and color harmony. Try it sometime!