Bitone - A flower with inner and outer segments of different tints of the same basic color. A bitone has lighter outer segments (sepals) and darker inner segments (petals). A Reverse Bitone has the darker tones on its outer ring (sepals) and lighter coloration on the inner segments (petals).
-- Definitions from the American Daylily Society
Pictured are some bicolors that I grow now or have grown in the past to show the variation of colors.