Different clothes have a different SPF -- most T-shirts have an SPF of about 4, for example. Red clothes are reported to let more UV through than blue clothes, meaning you're more likely to burn through a red shirt than a blue shirt. Sadly, I can't find the article I read that claimed that the color made a difference to give you a citation. A quick stroll through .edu sites for "sun protective clothing" turns up several comments that dark colors are best (along with tightness of weave and fabric thickness), but nothing specifically blue vs. red.