Leonardo: an open source contrast-based color generator. Create beautiful, accessible, and adaptive color systems using contrast-ratio based generated colors.
I love what this is trying to do and am in full support of the effort. However, it's a bit complicated – maybe offering up too much information. I really cannot figure out how to use it, and have given up. Would recommend simplifying and making much more intuitive to use. Also consider allowing the user to paste hex values into the color selector.