If it is starting at the top of your plants, it is usually attributed to a deficiency of one or more trace elements, and in this case, probably iron.
Giving a dose or three of soluble trace elements, available from any garden store, should fix this problem rather quickly and with the leaves recovering their "green-ness". Just follow the instructions, giving more will not make your plant healthier sooner, it is a gradual fix, just as the problem was a gradual occurrence.
Hope this helps, Organoman.