as u get to the end of the grow/flowering cycle u want less and less nitrogen and more p and k (npk )
less nitrogen means less green
its natural for the bottom leaves to turn yellow at the end
copied from a website
[FONT=Arial, Helvetica, sans-serif]What does N-P-K stand for?[/FONT]
N = Nitrogen 7-9-5
Nitrogen is the first major element responsible for the vegetative growth of plants above ground. With a good supply, plants grow sturdily and mature rapidly, with rich, dark green foliage.
P = Phosphorus 7-9-5
The second major element in plant nutrition, phosphorus is essential for healthy growth, strong roots, fruit and flower development, and greater resistance to disease.
K = Potassium (Potash) 7-9-5 The third major plant nutrient, potassium oxide is essential for the development of strong plants. It helps plants to resist diseases, protects them from the cold and protects during dry weather by preventing excessive water loss.