You don't have anything to worry about here, this stage is when the plant starts developing yellow leaves, it is displaying signs of 'autumning off' this is when the plant draws all the resources and nutrients (nitrogen and chlorophyll) from the leaves to develop bigger buds, the plant is concentrating on its further survival and knows innately its brief life is coming to an end, it is the sign your plant is at the final stage of its development and your juicy buds will soon be ready for harvest. You should also notice the pistols on the buds will be falling down, this is also a sign harvest is near, time for a pre harvest toke imo, you deserve it. Wp