i want to think heat and too much plant food, even though i don't know your grow details, but that's what happened to my plants when i overfed them...notice how the top layers of leaves are brighter than the others, even yellow some...the red is probably heat, how close are your lights and what kind/wattage are they?...i always saw yellowing (on newer growth esp.) when i fed my plants with nutrients and not just water, but zandor's right, some details would help much better...zandor will probably help better with some details than i would lol