ballast electrical question

[hillegom]

I have a 400 watt MH ballast that I would like to use for a 250 watt bulb.
I was told by an electrician that that is possible to do. As long as I do not use a bulb larger than 400 watts everything will work properly.
So he is saying I could run a 175 watt as well.
What do you guys think?
Would this change the kelvin of the bulb, not running the specified bulb wattage?
Would this waste electricity?

Thanks