me262 is correct. Engine is probably running too lean. But there is another option. If your bike is equipped with an air injection system, to lower emissions, which a 1979 should not be..., this makes the headers run hotter and blue. Another guess, is that the pipe manufacturer did not use a double-wall design, which usually solves this problem, and you may be able to return them... the reputation of the pipe manufacturer should be strongly considered, when purchasing pipes, as if a problem as this arises, they should be willing to back-up their product with a replacement or a refund.
Most likely your bike is running too lean. Most bikes run lean from factory. When you put on new pipes that allow for freer air flow they lean the bike out further. Either rejet or at least adjust the air/fuel mixture screws on the carbs.
All single wall pipes will turn blue. It's how far they are turning blue that counts. If they are blue a bit past the first bend, you are probably running just fine. If the pipes blue down to the second bend, you are certainly lean. Adjust jetting accordingly.
Air-fuel mixture screws aka pilot screws are for adjusting the air/fuel at idle and for use during carb syncronization. Correct me if I'm wrong but you have to adjust main jet size to affect the lean or rich condition at off idle.
You can adjust off-idle by just lifting the jet needle by resetting the clip to a lower position on the needle, which should have 3 to 5 slots. If that doesn't richen the mixture enough then go to a larger main jet.
Good posts here except that I'm not sure you really have a problem other than the pipes you installed are probably single-walled and are turning blue, which single-walled pipes will do. Straw-collered would indicate richer mixture, but ??
Does the bike pop on deceleration? Plugs look white? They should be tan in color. Does it pick up with hesitation off idle from a stop?
If you have no problems with any of the above then the bike is running OK. If any of the conditions exist then you may need to adjust the carbs. First try raising the jet needles one notch. Next would be to change the main jets to a larger size.
I traded an '81 Suzuki GS650 for a Yamaha in about '87 and only got $100. Dumb move.
There is a ceramic coating you can buy in a kit to line the inside of the pipes, it's sold for classic bikes to keep the chrome, uh, chrome. It's a pain to use (think gas tank liner, only for your exhaust headers and you get the idea) but if you desperately hate blueing its the only way to stop it.
Check your jetting FIRST, severe blueing is definitely a lean-fuel condition. Then you can decide if you want to line your pipes.