yellow =lots o' growth, but bad color 10K
white = some growth, semi-good color 14 or 15K
blue = minimal growth, color that popos. 20K
Granted this all depends on the make/brand. Some lights are barely even blue at 20k, and some are start white at 10K.
What may work best though is MH supplimented with PCs or VHOs. You get the best of growth, but you can suppliment to adjust to the color you want.
You may also want to visit Sanjay's site, he is the lighting guru:
http://www.reeflightinginfo.arvixe.com/