Seriously, header temperature will change as conditions change. A good example is with late timing. The gas is burning as it exits the exhaust port more than in the closed valve chamber. The same happens if the mixture is too lean. If you have either problem, it can be readily seen at night or in a very dark garage. The headers will actually glow red at the cylinder head. In cases where the timing is retarded, you can actually see the glow disappear as you advance the distributor.
Individual header tubes can also vary in temperature due to deficiencies in the intake manifold where fuel distribution is not even. People used to have to do goofy cross jetting to overcome this problem. 360 degree intakes do a lot to solve the problem over 180 degree intakes so you don't see this much any more.
Serious racing engines and factory R&D engines are run on the dyno with thermo couples placed in the header tubes about 2-inches from the header flange and they direct exhaust gas temperature (EGT) to individual pyrometers. This how they work out the intake system and proper timing. You won't find too many dyno shops with the pyrometer set ups because they are so expensive. Big diesel trucks use them on the exhaust outlet because they are also tuned by measuring the EGT.
I hope I didn't muddy the waters!
Pat
