They do go yellow as they start to dry off. I think they have finished their growth at this stage and the bulbs start forming their skins and drying out. They often get rusty too but I find that has no effect on the finished product.
I can't wait to harvest mine, they seem huge, if the stems are anything to go by. Some of mine were stolen from my plot! So I won't have the full amount, but still, plenty left.