hdtran
Geography Police
Damselfish:black is slimming...
actually, one reason I've heard is that the black dyes are most stable and last best, while colors tend to fade.
That's pretty much correct for rubber & plastics. The black in them is from carbon black (also in Neoprene, but I don't know about the lycra threads). Carbon black is essentially graphite, which is great at blocking UV. Sunlight (UV) is wonderful at breaking the bonds in polymers, which would degrade rubber/plastics/etc.
If you're underwater, though, the water will block the UV for you
That said, you can easily get very colorful wetsuits if you go to a custom maker (and they're not all that expensive). Liquidfit & Aquaflite (disclosure: I own an Aquaflite) will do colors for you. And no, you really won't look like lunch! Lunch to a shark is definitely not a neon-yellow diver with hot-pink accents! (Now, if you dress in brown to impersonate a sea lion...)
Finally, I should warn you that purple turns into blue at depth. In fact, just about anything (except black) turns into either blue or black at depth