Supernatural "Stay Green" will change the color. But you didnt ask the important question: Why are your leaves yellowing? Did you do a reservior change and move from grow to flowering nutrient mix? What nutrients do you use? At what levels? Dirt farmer? Rockwool? Grodan? See the more information you provide the easier it is to give an answer which helps. This is sort of like a post saying,
"help! my plants dont look like I want them to - what should I do?"
To be able to diagnose the problem and offer a solution it really helps so much to have some data - more the better. My first instinct is to go with the obvious answer which is nitrogen defficiency but it could be ph is way off so you have nute lock or temp or you over-fed them and they are dying and need to be flushed or Magnesium defficiency or insufficient lighting or exposure to light during dark hours or root pests or diseases or root bound or
a photoxic reaction to a foliar you used incorrectly to the water you use or
about 100 other things. To get a more accurate answer provide more information. Everything from room temp to nute solution ph to your choice of grow medium to your fertizlier type to your fert dosage levels.... you get the picture.
But yellowing leaves is commonly caused by nitrogen defficiency.
Al