Color theory principles such as complementary colors, near complements, and the interplay different RGB colorspaces have with one another play a key role in validating web color choices. In the current environment of ADA Guidelines playing such a large role in web design, I find a tool like the WAVE accessibility evaluation tool to be invaluable.
Using the contrast level ratio threshold of 4.5:1, the tool evaluates your site, page by page, to look for contrast errors and areas for improvement. Getting passing marks in the WAVE tool provides the first steps to getting the site customer-ready for UAT testing on a development site before go-live.