Color can be added to a pencil drawing with a variety of mediums: watercolor, gouache, ink, or colored pencil. When mixing color with graphite, first decide which is the most important ingredient: the pencil drawing or the color?
Is the purpose of the drawing to work out the composition of a piece in which all of the detail and rendering will be described through color? Or is the pencil the primary texture of the piece that is being lightly embellished by color? The two elements need to work together and not compete for the attention of the viewer—that could result in a murky image.