Yellow is usually a sign of a N lock up though not caused by high N alone. Alot of times when the soil is to wet and the bacteria can not convert the N (amminiom nitrogen) into a usable form (ammonium nitrate) do to the lack of oxygen. The pH then rises (that and the N reacts with O) and the plants can not get yellow though not before the roots are cut off from getting the oxygen into the plant to use the N stored up in it I would think. Usually comes after the leave start drooping.