ColorBrewer: Color Advice for Maps by Cynthia Brewer, Mark Harrower, Pennsylvania State University. Link.
Subtleties of Color (Part 1 to 6): Introduction and recommendations for the use of color in visualizations by Robert Simmon, NASA Earth Observatory. Link to Part 1.
Color Oracle – Design for the Color Vision Impaired: free color blindness simulator by Bernhard Jenny, Oregon State University. Link.