Web accessibility also includes catering to sighted visitors that happen to be color blind. When you design with color blindness in mind, aim to have your website functional without using color for distinction.
Color Blindness Simulator - etre Limited
This free simulator will check an image to show you how a colorblind user would see it. You can upload a JPEG image to see how color blind users may see it.
Color Check - etre Limited
This free online tool determines the color difference and contrast between any two colors and whether or not the two colors have enough contrast and brightness.
Creating Accessible Images - WebAIM
This article shows how to use color, contrast, and other techniques to create an image accessible to people with color blindness.
Colors for the Blind - Toldeo-Bend
This site offers basic color-blindness demonstrations and explanations, but also suggests color palettes that should be viewable by most forms of color blindness.