21,000K would be wayyyyy blue. I used to be into growing coral, and I'd use metal halide bulbs in that spectrum for growing lots of stony colonies. They aren't even that great of bulbs IMO, I used them to flouresce corals without supplemental actinic lighting.
scagster Reviewed by scagster on . which bulb is superior which of these bulbs are better Grolux 400w Dual Spectrum Sunmaster 400w Son T Plus Rating: 5