A free program for Chrome, by Level Access.

Accessible Color Picker is an application for the development of accessible web pages. It is intended to make the process of color contrast testing easier for developers and to help them make their websites more accessible to people with low vision or color blindness. 

You can choose the foreground and background colors you want, and the program will calculate their color contrast ratio. If your colors do not have sufficient contrast, this tool will suggest similar colors that are compliant.

Other features: