The higher the K rating the more "blue" it will be, that being said, corals need a mixture of both to get optimal growth and colour.
I have a T5 setup which has 4 bulbs
2 x 12k
2 x 20k True Blue Actinic
I think with MH bulbs most people use them for the 14K range and then use the Blue T5's for moonlights/growth as well.
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