Color is often used in web design for grouping related items, denoting required fields on a form, or distinguishing one piece of text from another, e.g., hyperlinks. Color enhances the aesthetics, usability, and even the accessibility of a website. However, some users have difficulty perceiving color so, when color is used, it must not be the only means of conveying information.
Note that it is important to assure adequate contrast/luminosity of color, too, so be sure to consider both color and contrast together.
If color is used on a website to convey information, such as indicating required fields, hyperlinks, active elements, or other information, there needs to be another, secondary method of conveying the same information. For example, required fields could be marked with a red asterisk; hyperlinks could be blue and underlined; and active elements could change color and be marked with a heavy border.
To check whether color alone is being used to convey information, visually inspect the pages to find all instances of color usage, and if the use of color is meaningful, ensure that there is a secondary means of conveying the same information. Also look for references to color in the content of the pages, and make sure that these references describe where to find the information. For example, you might include some of the text and/or the page location so all users will be able to identify the information.