You won't need supplementation for color, but you may consider a different combination if you want to put a clam at the bottom of a 21" deep tank with 150 hqi's.
Natural sunlight has a kelvin rating of around 6,500 K
. The higher the kelvin, the bluer the light and less red. 20K is pretty blue and brings out coral and fish colors but isn't the best for growth. If you want higher growth and a better ability to keep sps
, clams, anemones; then you may want to go with 10K metal halides and add your color by supplementing with VHO
A third option is to split the difference and go with a 14 or 15K lamp. It won't be overly blue, but you should have better growth without having to add aditional lighting. A couple good choices would be the phoenix 14k or XM 15K.