What I would like to know is if I go to a higher rated Kelvin for the MH
will it affect the plants too greatly? And what rating do people suggest?

Anything on up to around 12000K should be OK. 20KK marine reef bulbs
might not have enough red for some plants, although lower-light plants
would do fine.

The 5500K MH throws off a yellowish/ orange colour and Id like to have a
more whiter colour in the tank, as it sometimes looks like the plants are

I found some 6500K bulbs to give too green of an effect, what with all the
greenery in the tank. But your tastes may vary. 10KK bulbs definitely
will not be too green, but as was posted the actual color effect can vary
among the brands and what ballast is driving them. Some are blue-white
but others purplish, containing more reds with the blue. You might find
some "trial-priced" bulbs on Ebay or the reef sites.

Mike