it sounds to me like you're just seeing accelerated growth. when they begin to grow quickly, the base of new growth will have a yellowish look while the "grown out" portion is green. as the leaf grows out, the entire leaf greens up.
if you have a general yellowing, add nitrogen. if yellowing begins at the veins and works outward, use cal/mag. if all you're noticing is problems with tips that grow out of it, continue as you are right now.