How close is the HPS to your plants? I would start it a bit higher than the 150 and slowly lower it until the plants are used to it.

It's like going from a warm day to a walk on the sun! (well, not that bad - but you get the idea)

This may not even be the problem, but give some input

Where are the plants yellowing, do you have pics, what's your PH, how many plants, what strain, info info info!