These things are really smart for long days in the sun. You put them on before going outside, then apply sunscreen, completely covering the band. When you go out in the sun it turns bright purple. The band will start to fade the longer you stay out - you reapply sunscreen when it turns lavender. When the band turns almost white - it's time to go in.
For someone with super pale skin who burns after being outside for a few minutes, these things are a lifesaver. I've spent 6 hours out in the blazing sun and been totally fine. Definitely a summer staple.