Even for those of us who love color, every once in a while it's fun to experiment by painting with one color only. Using just one color really helps you see your painting in terms of lights and darks. I find doing this helps me establish values when I carry what I've learned over to my usual, multicolor work.
Sepia is a great color to use for this exercise. Not only does it evoke old-time photography, it also can be used in a range from very dark, almost black, to subtle, minky shades.
Here are two small paintings I've done using only sepia. Both are from old family photos; the one at the top is a quick sketch done without any pencil drawing. The bottom one may not be quite finished.
Have you tried this? What do you think of using just one color?