Are you sure you saw a golden Chu? I thought so too but it turned out to be a yellow Chu and when you scoop it up in a bottle it's used as lantern oil.
Purple Chus seem to come when Chus mix, at least from what I saw in the Cave of Ordeals. (I would guess that if the scooped up purple Chu jelly was predominantly blue, it'd heal you all the way, and if some lantern oil was mixed in, it would hurt. Ick. That's pure speculation of the "logic" behind it, of course, though, most likely it's just totally random. I (or rather, Link) drank purple Chu jelly when I (Link) had a quarter of a heart and it "hurt", but it didn't kill me (or Link, rather), much like how in Star Fox 64 Andross can't actually chew you to death.