Best stuff you can possibly buy to restore pipes is Blue Job.  I was soooo skeptical when I ordered it, but WOW.  The difference is amazing.  Check ebay first cause there's usually decent deals on there for it.  Takes a little elbow grease, but the results are amazing.

I haven't used Blue Job, so I can't comment much on it.  I have repeatedly seen the words "elbow grease" associated with it though.  I stumbled over this product at NAPA:


I kid you not, this wipes the blue off of my pipes with about as much effort as wiping up water with a paper towell.  I think the can cost 5 or 10 bucks.  It is a big chunk of wadding saturated with some kind of polish.  Just tear a piece off and wipe it on.  Then, wipe it off with a soft rag.  It does not prevent re-bluing, however.  Tuning my carb did that.  Blue Job might help with that issue?  Anyway, I would heartilly recommend trying Nevr-Dull.  It makes this chore a snap.

Good Luck,
Jason

blue job nor never dull worked for any of my pipes , another cause for discoloration is a small exhaust leak . I recently painted my exhaust with 2000 degree heat resistant black paint picked it up at autozone for 6 bucks a can i sanded the exhaust down to roughen it up then put about 8 coats on the outside and 3 coats on the inside and let the exhaust sit for about 7 to 10 days to help cure i could've put it in the oven for 2 hours at 200 degrees but wife would've killed me . that is my input on the discolored pipe . good luck