Power Rangers Movie Clip Underwater
 

"Check out how we glow!" Very next scene they're no longer glowing.

Also, how do you glow black?

Purple?

That's purple, though. If he was glowing purple, why didn't he say "I'm purple?"

It's really just a poor choice of words on Jason's part. They're not really glowing so much as giving off individual colors into the water; Zacks notices the color he's emitting is black - which is possible even if glowing black isn't really a thing.