Ah, it was a very, very popular idea in Zutaran circles after "The Cave of Two Lovers" aired, with lots of folks getting way way too fixated on the color symbolism of their clothes in the paintings and all that. At this point in the game it looks like any such symbolism was purely unintentional, but still, it can be a fun thing to play with in fic.

And of course coming up with your own fanon past lives can be just as much fun, too -- the Sozin-and-wife thing is a neat idea, some folks were pointing out that one of the faces Koh uses, of a woman loved by the Avatar in a previous life, also looks a little bit like an older Katara; and Vicki So in particular, in her lengthy fic series, comes up with an interesting original take on things that I can't go into much without spoiling the stories for you. (Download them for plane reading or something, they're awesome!)