The first project in my Intro to Illustration class this semester calls for incorporating a "toned" palette. Some of my students asked for a more in depth explanation of how this is done. One of my all time favorite descriptions on how to do this is contained in the long out of print Creative Illustration by Andrew Loomis. His brief but meaty description on how to incorporate a toned palette into your work is above. Notice that only one color in the mix stays pure- the toner color itself. As you mix in the chosen color, all the other colors shift toward that color, thus creating a unifying effect.
The key to creating color unity in a toned palette is to not overdo the amount mixed into the various other colors. To heavy a hand will result in muddy, monochromatic color. You will notice the influence of the toner color in the skin tones of each of Loomis' examples, yet the flesh tones feel totally natural in context. I frequently use a "toner color" to unify my color scheme. I think Creative Illustration is a must read for any illustrator, or artist for that matter. I found my copy of for $30 about eighteen years ago after a lengthy search. It's still available, but no longer so affordable. If you can't find or afford it, PDF facsimiles are available online as well.