This extension is based on the idea that people need to know if they are going to see colors and dark elements (dark text, buttons, images, etc.) on their screen. If the contrast is high, the colors will be clear and easy to read.

This is because some web pages can contain many colors, and you need to set a color to check the contrast of all of them. 

This is a color contrast checker that is based on the W3C Web Accessibility guidelines and is compliant with W3C Web Content Accessibility Guidelines (WCAG). The extension checks the contrast between the first plane and the background of the page and gives you an estimation of the contrast of all elements on the page.