Adding HPS to a white grow light will result in your plants getting more red photons which tend to lead to taller plants. By adding more red light in flower, you're helping to decrease the loss in yield that's caused by lights that have blue photons.
Light color influences plant shape; the quantity of photons influences yield. The results of a paper published in 5/21 show that yield increased on a linear basis, per below. The ratio of flower to foliage ("crop quality") also increased with increase in the amount of light.