Okay, so the players try chopping off a head, say the red one, and the whispers start to dissipate (and no heads grow back). Awesome! they think, and quickly dispatch the other two. But once all three heads are gone, five more grow back: green, blue, red, white, and different red, I guess? And the party is immediately beset by visions, which are somewhat clearer and more vivid than the last ones.

Green dragons carry luma (Y), blue dragons carry blue minus luma and red (not that red, the other one) carry red minus luma

Unless the dragon is Mono in which case I think red would be video.

when the gold head speaks, its victims see visions

This is what writers and literature enjoyers call imagery.

DVI-amat?

Bahdmi-amut?