All green plants produce their own food from water, CO2, and light, by a process named photosynthesis. The chlorophyll molecule, which is essential to photosynthesis, is coloured green and contains one atom of magnesium. So adequate magnesium is absolutely vital to the growth of green plants.
If there is a shortage of magnesium in the plant´s nutritional intake, an early indication is the yellowing of leaves. It is easy to remedy any deficiency by feeding a dilute solution of Epsom salts, MgSO4, in a concentration of about a teaspoon to a gallon, two or three times in the growing season; a little goes a long way.
Magnesium deficiency is distressingly common, many commercial soils contain little or none of the element, and many ferts only contain tiny amounts of magnesium, if they contain any at all. And growers do not always appreciate its importance.
But magnesium deficiency is not the only cause of leaf yellowing - a shortage of nitrogen can have similar symptoms, but this is MUCH less common as most of us routinely feed nitrogen-containing ferts. Some pests, such as red spider mite, can cause leaf yellowing, as can a whole host of viruses to which your ladies are susceptible. And don´t forget it is normal for the lowest leaves on the plant to yellow and die. They have done their job, and your buds are now being fed by the products of the photosynthesis being carried out by the fan leaves.
But if you do have a problem, I do recommend that you try some Epsom salts in the first instance. It may well not be the answer for your problem, but it very often is.