Your buds are filling in well, but the fact that leaves are yellowing is not a good thing. The more healthy green leaves your plant has the better it processes light, does what plants do, and that means more bud. I can't tell you what it is, but you should always strive to keep every leaf on you plant as healthy and green as possible.
Someone said earlier "thats what leaves do when you flower" and I would have to disagree with that. That happens as a result of some sort of deficiency, not because your plant has begun flowering. Other than that the bud is looking good, and I hope you get a great harvest from her.
I too am using one of those Steralite cabs. I think it is a pretty cool, budget grow cab. Good Luck!