At paintingfriends.com, we are just beginning to study colors that we currently have collected during our time as painters. We all own so many different tubes of colors and the only way to get a handle on what colors work best with others is to do color studies, swatches and wheels. Also, we believe this will help us to break away from the palette we most often use and try different colors that normally sit in the drawer or bin.
I have been to handprint.com to check out their watercolor visual and mixing complements and to danielsmith.com to see their recommended complements, because that is the brand I use most frequently. One of the things I noticed is that some of the complementary greens are mixed colors (having more than one pigment). It is also interesting to note which colors are transparent, semi-transparent, opaque, granulating, staining or non-staining and especially important is the pigment lightfastness.
My personal preference is to use a light fast rating of excellent or LF1 with a preference for transparent colors unless I am painting a landscape. The earth tones used are mostly raw sienna, yellow ochre, (opaque or semi-transparent) or quin. gold (transparent) along with burnt sienna, sepia, transparent brown oxide, transparent red oxide, and quin. burnt orange. I also use cerulean blue a lot to mix greens in landscapes. There is something about the muted mixture that appeals to me.