light spectrum. the brand also makes a difference. some 10kk bulbs are more blue than others etc.

i run 14kk coralvue SE bulbs which are somewhat blue, but not as blue as XM 15kk and certainly not as blue as a 20kk bulb. i also run 10kk coralvue reeflux DE bulbs which are almose pure white. best thing to do would be see some tanks with different bulbs to decide the coloration you like.