A significant percentage of map users have impairments in distinguishing color.
I would like to highlight an excellent article on colorblind friendly cartography:
Jenny Bernhard and Nathaniel Vaughn Kelso, 2007
The presented techniques help the cartographer produce maps that are easy to read for those with color vision impairments and can still look good for those with normal color vision.
The authors also developed a handy colorblind emulation tool: Color Oracle.
Color Oracle is a free color blindness simulator for Window, Mac and Linux. It takes the guesswork out of designing for color blindness by showing you in real time what people with common color vision impairments will see.
No doubt many of you have heard of Cynthia Brewer's excellent tool for choropleth maps. Note that a a "colorblind safe" filter is included.
Though not dedicated to mapping, this subredditt includes resources and examples of both good and truly inaccessible designs. Useful for getting feedback on drafts and seeing general color styles to avoid
1) Color Design for the Color Vision Impaired. (2007). Cartographic Perspectives, 58, 61-67. Retrieved December 10, 2014, from http://colororacle.org/resources/2007_JennyKelso_ColorDesign_lores.pdf