Are you using a bloom enhancer like Big Bud or Kool Bloom? I've had these cause increased yellowing during that later stages of flowering. As long as you don't over do it these products can do great things for your flowers. Just be sure to give them a good flush because those bloom enhancers will leave a nasty flavor in the final flowers.
One thing to be careful of is BUD ROT. It's ugly stuff. If you get some really giant flowers going, take a tooth pick and pry them open a little bit on different parts of the buds. This will allow air circulation throughout the flowers and stop botrytis (bud rot). One a couple occasions I've had flowers the size of my forearm and when they were harvest I found out that the insides were grey and crumbled from botrytis. That little toothpick trick was given to me by an veteran grower and I have never had bud rot ever since.