The most common type of color deficiency is red-green.
The "Vischeck" color blindness simulator is a useful tool that enables you to view a copy of your Web page as it would appear to someone with red-green color deficiency.
Vischeck is a way of showing you what things look like to someone who is color blind. You can try Vischeck online - either run Vischeck on your own image files or run Vischeck on a Web page. You can also download programs to let you run it on your own computer.