About 8% of all men experience some sort of colorblindness. It’s very important to be aware of this audience when you are developing websites and learning applications. Those of us without color vision deficiencies are often unaware when we use color in our designs to signify levels of importance.
A useful tool to use when designing your sites is the Colorblind Web Page Filter. This tool takes an URL as an input and renders the target page as it will appear to people with varying types of colorblindness.