It *could* be a sign of parasites or infection. It *could* be nothing of the sort. Are their fins clamped? Or do they seem to be scratching on objects in the tank? Any other changes in behavior/coloration? And, as always, check your water parameters.
Look closely at the reddened areas and see if there is anything there that doesn't look like it belongs.
They may be displaying their breeding colors? What species are your danios?