Color changing rats?

I accidently ran myself out of rats this summer~ So I built myself another rack and purchased several medium sized rats from a guy in Huntington Beach.

I got these rats two weeks ago tomarrow and I loaded up this new rack. My old rats were having a rather nasty outbreak of the sniffles, so I put my old rats, my mice and these new rats on Tetracycaline hydrochloride. I did not quarintine as I figured it was MY rats that would make the new rats sick~ and it would be best to expose them to the current strain of myco and let them get over it right away. No problems. My rats seem to be much better, the mice are fine~

The new rats are changing colors. Not all of them. Just the greys. And not all of the rat. It starts at the head and runs in what looks like a skunk stripe down the entire body. I will attatch pics. The hair that has darkened is shorter than the lighter hair, but not thinner. It does not seem to be happening to the black rats~ just the grey ones~ but I am SURE they did not look like this two weeks ago~ so whatever is going on is happening since they got here. I don't see any evidence of anything like this in my old rats either~ but I don't happen to have any greys in my old colony just now. It does not seem to effect the rats health at all~ but I only noticed it this morning so only time will tell.

Anyone seen anything like this before?

Couple more pics~
I'll have to load them one at time as I don't have access to my PC and Web page right now.

This one is a different rat~ only her head seems to have changed colors. I've got about 5 to 7 grey rats that are changing colors.

Any ideas?

This is just a guess, and it may not seem very plausible considering all of the factors involved, but I've seen stranger. Could they be shedding? I only ask because the moment I saw the first pic you posted it reminded me of my German Shepard and how she sheds. It starts at the head and works its way down the rest of the body. After a week or two I have what seems to be a two tone dog with the longer hair on her bottom half.
Why it would happen to just the gray rats is anyone's guess, but strange as it may sound, perhaps your gray rats have something genetic going on (certain allergies and immunities can be genetic) perhaps the medicine has something to do with it. Or perhaps the gray rats were/are more predisposed to stress than the others and with their recent move they are exhibiting it by shedding.

I know this is long and perhaps I'm stretching it a bit. I'm willing to admit it.