Do you think all the birds lay very pale yolks sometimes, or is it just one or two?
If its all of them, this could point to a nutritional deficiency, but if just one or two, the basic food is probably OK and there is something wrong with hens concerned. The usual advice for getting a good coloured yolk is to feed dark coloured greens, such as cabbage as Chris says, (not lettuce which is nutritionally pretty useless, although they like it) and also as much free ranging as possible where they can eat short grass and find a variety of other plants. Good quality pellets often contain marigold petals as a yolk enhancer, which is also excellent. You might also try a broad-spectrum tonic, preferably one which includes seaweed.