Leaf yellowing is normal, that is part of the life cycle of the cannabis plant. In its native habitat, you can observe exactly the same thing in the surrounding foliage of many plant species. In fact, I never look at trichomes on outdoor plants, I look at the degree of leaf yellowing. When the top fan leaves start to yellow out, it is time to harvest for the simple reason that the plant is not going to be putting on any more weight at that point.