In my experience, the yellowing tips means that you are maxing out the nutrients for the plants and it is beginning to burn them some. Many of us push our plants pretty hard and get some tip burning. The important thing is to back off the nutes some at this point. However, if you are getting a lot of yellowing across the plant, but only on the bigger fan leaves and you are well into flowering, then it may be the final yellowing off that occurs a lot of times. Is the yellowing only at the tips of the leaves or is it more uniform across the leaves but beginning at the tips as that will tell you which it is.
In my experience, the Kush tends to make smaller buds than some of the other plants like Big Bud.