A. Don't use a filter on any light, any time...:stoned:

B. Metal Halide bulbs come in different Kelvin temperatures. 3000k, 4000k, 5000k, 6500k, 7200k.
3000k-4000k (warm white) works well for flowering. 6500k-7200k (cool white) works well for vegging. Just swap bulbs...

HPS bulbs are mostly 2000k-2400k. They have more lumens; which can help with some sativa strains.

A 400 MH bulb can handle 3-5 flowering female plants, very well.