They will if you're not moving and they aren't getting any air.

Odd then that this is the first time I've noticed this in 95,000 miles.
Checking valve clearance and compression at the moment then will try blocking off the AIS.

But then again you don't normally get to see the headers when you're at full throttle for any length of time - as you'd be going quite quickly!

No sir, they DON'T all do that.
 

I'm very observant then!

Hmmm?.....blocked the AIS but still no better..engine definitely hotter as well.

funny you should say that...
I have just noticed that the right
pipe is getting hotter than the left..

I have just put a metal mule 2 into 1 on
with the colour change from new the right
pipe is getting hotter than the left.. As the
right pipe is a much deeper blue gold colour...

Pic below

The colour difference is more than likely due to the different length headers (longer left - cooler) and from your photo they are a different thickness, the right one having a and overlap and a weld in it too. The right one also has a much tighter bend in it near the cylinder head too, therfore the metal will be thinner at that point on the outside of the bend radius. I'd suggest it is the pipe design causing this, rather than the way your motor is running?

I've just fitted a single MTC silencer so can now smell that the bike is running rich - so I'm assuming that the O2 sensor has died and is causing the ECU to run in "safe" mode. The resultant overfuelling would presumably be burning in the headers causing the glow???