Wow. Nova on a stick. OP.
Not exactly, the cost of playing the card and the different element required to use the effect combined with summoning sickness (no instant quantum) and possible CC from your opponent keeps this from being overpowered.
this definitely works better as a permanent, but instead of sun for upped, you could call it sun ray.
I think the concept of using water as a means of rainbow quanta production seems pretty ok, though i dont think a sun can qualify as a creature. which is why it should be a permanent.
overall its good in concept but in mechanics it only just replicates nova.
besides , there is no black or white in a rainbow. those are shades.
hence 
dont fit into the concept.
I agree, making it a permanent would have made a bit more sense...However, if it was a permanent then you could use enchant artifact and you would have an unending supply of quantum. Even immortal creatures can be affected by things like the fire buckler. What could get rid of a sun, if it was a permament?
A Light card losing HP?! Let's get an Archangel out here...
The point of it being light is so you can't heal it
If you're using a rainbow deck you couldn't have too many angels or else you're quanta wouldn't be balanced. Too many angels and you wouldn't have enough light quantum left.
