it depends on what you like. bluer or whiter ?
how much can you deall with algae, whiter = more algal growth given nutrition is there.
and if you are after color, or just growth.
with 6 bulbs, I would do 3 blue [ati or KZ]
2 white [10000K or 140000K]
and 1 fiji purple KZ.
that would give u a good starting point, then if you want it whiter, swap another bluw ith white or other way around.
I usually get extra bulbs, andplay around with them,. and its not a waste as you will need to replace the bulb soon anyways.[8-12 months]
HTH,