Last year, I published a blog called, Surround yourself with color. As long as it's gray! In the blog, I discussed how important viewing conditions were when evaluating color, and how surrounding colors affect how we see color.
Recently I came across a great website from the laboratory of Dale Purves, M.D. in the Center for Cognitive Neuroscience at Duke University. There are many interesting optical illusions that deal with color constancy, chromatic adaptation, color contrast, brightness contrast and other illusions that play tricks on the eye. All the illusions are interactive, allowing the viewer to mask off or adjust brightness on different parts of the image.
These exercises underscore the importance of maintaining proper viewing conditions when making critical color decisions. Give it a look. You won't believe your eyes!