Here is an awesome product that resides in the quiver of many creative developers. If you have not heard of ColorSchemer, you could be missing out on designing with harmony.
ColorSchemer Studio 2
ColorSchemer Studio 2 is a professional color matching application for anyone from hobbyists to advanced professionals. Work with a dynamic visual color wheel, instantly explore harmony relationships and even let ColorSchemer Studio intelligently suggest color schemes for you!
Try ColorSchemer Online, free.