Why it matters: Having a product that is accessible is not a plus — it’s a must. So, relying solely on color to convey hierarchy, content or functionality means that people with visual disabilities, such as color blindness, will not be able to use your product properly and will be an excluded demographic group.
How to test it: colorfilter.wickline.org puts a color filter on top of your webpage and test it for different kinds of color blindnesses. You can also screen–capture your product and convert it to grayscale in an image editor to see whether you can tell colors apart easily.
Look at this example: