I have a MH fixture with 2 150 watt bulbs. They are a year old and I guees need to be replaced. Money is a bit tight. How bad would it be to replace only one now and the other soon?
Of course it would be better to change them both out, but if you can't, one is better than none.
The color thing is just during the break-in period for the bulb.
The intensity of the old bulbs will be on their last leg but one is better than none just try to get the other replaced as soon as you can since you need both to supply light to both sides of the tank.