The redness of the combs vary for many reasons, heat, cold, stress, and whatever else. Watch for other issues combined with it, but by itself it's not an issue unless they have parasites of some kind and have become anemic from bloodloss. The red coloring comes from blood, so anything affecting circulation can affect the comb color. A certain amount of paleness is normal, watch for it on any one bird in particular for extended lengths of time. Mine seem to change color throughout the day. Flares up rainbow red when they're about to lay their egg for the day. Then it gets hot out and they go a little pale.
