Don’t forget the usefulness of a color wheel (or color circle) when looking for an additional color to use. The final image will look more vibrant if you can add some complementary colors. A complementary color is one that is opposite on the color wheel; these two colors, when mixed in proper proportions, will produce a neutral color (blacks, grays or whites) Or the image will harmonize and look more calming if tertiary colors are selected. Tertiary colors are when primary and secondary colors are mixed together. At least, that’s my color theory.
Sproatmeyer 