My name is ***** ***** I’ve been involved professionally with dogs in the health and behavioral fields for over 18 years. It will be my pleasure to work with you today.
Actually what turns the hair red is a dog's saliva though other bodily fluids might also have an effect. The saliva contains an enzyme that causes a chemical reaction on the hair turning it red. There are many products on the market to remove this. They are marketed as Dog tear Stain Removers but can be used wherever the hair is being discolored. The following site has a good chart that discusses these.
The website below has a different approach suggesting altering your dog's pH levels to prevent the staining which you may wish to look at.
There is also an antibiotic that your vet can prescribe called Tylan that has been shown to be useful to stop saliva from discoloring a dog’s coat. So perhaps your dogs are licking their rears a little more than normal. If they are, then I'd have a parasite check done to ensure they don't have worms which might lead to licking the area more often. Since it is the saliva and tears, longer hair on the face would tend to show a change as well.
I would talk to your vet about possibly using an antibiotic on them to stop the discoloration. I hope this information is helpful to you. If you need more information or clarification, please reply and I'll get back to you as soon as possible. If you are satisfied, please take the opportunity to rate.