I'm working on a wedding portrait. When I am preparing to start a new painting, I choose a group of paints that I plan to use and spend some time mixing the pigments to see what mixtures are possible. After I settle on the mixtures I want I usually make notes in my sketchbook about the proportions I used. This helps me re-mix the colors if I need them again for future paintings.
If I want to experiment with a wider range of pigments, as in this wedding portrait, then I make a color chart. This is a more tedious process but it ensures that I try all possible combinations and leaves me with an easy-to-read chart that I can refer to later.
This chart took a couple of hours to complete. Even though this chart is a means to an ends for me, it helps me understand how artists like Josef Albers could make a career out of exploring the interactions of color combinations. There is something peaceful about seeing related colors organized on a grid.
For my painting I need three groups of colors: reddish brown, yellowish brown, and gray. I chose Cadmium Red + Raw Sienna for the reddish brown and Burnt Sienna + Cadmium Yellow Deep for the yellowish brown. For gray I chose Sap Green + Venetian Red. I want a warm gray and a cool gray so one mixture has more red and the other more green. I mix lighter versions of each group so that I have a light to dark range within each group.
To keep my mixtures from drying out, I scoop them onto plastic dinner plates and keep them in a freezer. They will remain "wet" for months. This lets me work on other paintings without fear my paints will go to waste.