the 6000k bulb produce more PAR compare to 20000k so that is why your coral benefits more light spectrum from the 6000k bulb. By the way, your tank is 4.5ft so you will need at least 2x150watts to have a good coverage.
if you wish to use a 20000k bulb, what i suggest is to use atleast 2 x150 watts and run longer hours. Alternatively, use a 14k bulbs with T5 blue suppliment to have a blue tint effect.
my 2 cents...