hi, i had the same problem before, mine was due to the distance my light was from the plants if it is not a hps or metal halide get it inches away. Otherwise the plant strives to get closer to the light and then stretches
I DISCOVERED WHITE SPOTS ON MY LEAVES OVER A MONTH AGO, SINCE THEN I TOOK ABOUT 50 CUTTINGS AND THESE HAVE BEEN VEGGING FOR 2 WEEKS, THEY ARE ALL GOING REALLY YELLOW, I NOW KNOW THAT I HAVE SPIDER MITE COULD THE SPIDERMITE BE MAKING THE LEAVESGO YELLOW AND THE STEMS PURPLE...