The hen is supposed to get off the nest at least once a day to eat, drink and exercise. Ordinarily they do not eat and drink while sitting; they are in a bit of a trance. Occasionally, a broody will not get up and can actually die while sitting on eggs. They can develop joint problems from sitting still all that time if they do eat and drink enough to stay alive. Actually, I would suggest you move the food and water out of reach. They are also more prone to developing lice and mite infestation and this can make the comb pale if bad enough. A permethrin/pyrethrin spray or dust will be safe for mama and for chicks.
I take my broodies off the nest and make them move around at least twice a day. I don't think they lose as much weight while brooding, and this way I don't have to wrry whether I have one that won't get up. That few minutes won't hurt the eggs, and they go right back.
Another possibility on the pale comb is that she needs worming.
