Color blindness (color vision deficiency, or CVD) affects approximately 1 in 12 men (8%) and 1 in 200 women in the world. This is about 4.5% of the entire population, most of whom are male. (from http://www.colourblindawareness.org/colour-blindness/ )
ProfHacker included an article on Creating Color-Blind Accessible Figures recently.
There is tremendous variation in how individuals perceive and distinguish colors. These differences can be due to color vision deficiency or color blindness, as well as other medical conditions affecting the eyes or brain. Other factors such as device display settings, corrective lenses, and environmental lighting conditions can also affect the perception of color differences.
This article includes an interesting color-blind barrier-free pallette, which is worth taking a look at.