If it is past the 5th week of flower it's not too worrisome. Some yellowing is very normal as the plant nears the end of its life cycle. If it is before that, I just water ONCE with the grow fertilizer to slow down the process a bit. If you often have this problem, consider adding an organic N source to your soil, like a little bit of blood meal mixed in VERY VERY well.. not TOO much because too m uch N in flower leads to leafy growth and a harsh taste.