lol buffy
they are just different color spectrums
6500k is the color of daylight. 10000k is bluer 12000k is even bluer, as the kelvins get higher the light gets bluer.
as the suns rays penetrate the water the rays get filtered out. the blue lights penetrate deeper. the reds and greens get filtered out quicker the deeper the light penetrates. so with reef lighting we try to mimic the spectrum that a reef sees so we try to blend the lamps to acheive this.
thats why the water looks so blue in the reefs