Are you positive it wasn't like that the day before? Or maybe you just didn't notice? What breed and how old? Mature roosters often get a reddish wash down the sides of their legs. I see it in the BR, Buff Orp and the RIR roosters all the time when they reach maturity.
When I picked him uup the day before his legs looked fine, unless for whatever reason I just didn't notice it but tonight it's real obvious. I didn't see them but only to feed and water yesterday though so I don't know how his legs looked. He's 6 months old. I am looking through posts for the leg mites now... Seems there are a lot of different things to try.
Well, leg mites are easy to deal with. Coat his legs with oil, vaseline or spray with Pam cooking oil to smother the mites. Usually, those result in raised up scales. Red, inflamed legs can be fungal, too, but that might have nothing to do with this. He could be reaching sexual maturity. Treat him for mites just in case, though. Can't hurt!
I will feel silly if it is just hormones but will treat him anyway. better safe than sorry. I am not that experienced in chickens as you see, but they are my spoiled rotten little friends and anything that doesn't look right scares me.