Getting bright red combs
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How old are you chickens? Pullets usually develop red combs when they are reaching laying age. I have observed a change in comb colour over the course of a day in some of my hens.
If they are of laying age and their combs are not red, it could be down to a few things, but firstly I'd suggest adding some vitamin supplements / electrolytes to their water and ensure that you are feeding them proprietary feed with any "treats" constituting less than 10% of their daily feed. There may well be other causes, but this course of action will certainly help you rule out nutrient deficiencies.
All the best
Have you checked them for internal parasites, or mites and lice? Any of those could compromise their appearance and health. If you talk to others that show, or observe them, I wonder if they use a bit of Vaseline on the combs to bring out the shine.