5,5K is the color of sunlight. In theory that sounds like a good idea, right? But as Richard said, everything will look very yellow and brown. I ran 6.5K bulbs for a while. I got GREAT growth for them, but I didn't know there was such a thing as blue or purple corals under them. The lower K value bulbs are great in a propagation tank where your main concern is rapid growth, but not very good for color. 10K in my opinion is a good balance between growth and color.

Just another note in case you weren't aware of it. Generally, the lower the K value, the higher the PAR (photosyntetically available radiation). That is why you get better growth from low K bulbs like 5.5 and 6.5K bulbs. You can still use them, but you have to supplement with a lot of actinic (purple) light.