Hey Skydog,
I have seen that Big Tubes are notorious for discoloring, sometimes it will just happen because. It can be either from the water pump, water flow, or the way your tails dump the water into your exhaust. At 17PSI you are fine with you water pressure, lets see if you can restrict it back at the headers a bit more? Are you picking up water from your drive and an addition pickup? You might need a larger tube in your pickup? Has this always been a problem or is it just starting to happen?
One thing you can do is go to a 2 stage pump and pump the headers with there own source and restrict the engine off its own water dump. We have a few customers with carb engines with big headers that run them that way.
Give us a call and we can help work to resolve this problem! SS headers look way better when they are that nice shinny, polished color!
Eric