Complementary colors. Many artists think that mixing complementary colors creates mud, and that can happen, but I think combining the two creates beautiful neutrals. Using transparent watercolors can make it easier to keep your neutrals clean and not muddy. Because complementary colors intensify each other when used together, you can create some very dynamic paintings when you use them. I like to apply the colors to a wet painting and let the colors mix on the paper, instead of on my palette. Layering complementary colors can also be successful with the glazing method, drying each color before adding another. Here are some examples.
I prefer to mix my own greens, greys and blacks. A mixed color will be more personally yours, and it will have more depth, especially when applied to wet watercolor paper, as it will separate out into the colors you used when you mixed it. If I want an olive green I use a warm blue, such as Ultramarine Blue and a warm yellow such as Cadmium Yellow Medium or Deep. If I want a fresher green I would use a cool blue such as Thalo Blue and Lemon or Winsor Yellow. One of my favorite greys is Ultramarine Blue and Cadmium Red Light. Blacks can be made by combining the three primary colors: Red, blue and yellow. or Burnt Sienna and Ultramarine Blue. There are many combinations to try. Be sure to put your samples in your sketchbook and write down what colors you used so you can repeat the formula when you need it. Here are some the the greys I tried before I did this cloud painting.