Maybe none. Color is often indicative of the temperature seen by the tube and comes from the formation of a thin oxide passive film - the thickness of the surface film forms the color you see and is generally a function of highest temperature the tube saw.

Quote:

is generally a function of highest temperature the tube saw

shouldnt that read '...a function of highest temperature the tube saw, in combination with the medium'?

What is the material of the tubes. Are the tubes new or have they been in service and if in service what is the service medium?

XL83NL is correct. I held back because there has to be enough time at highest temperature for the film to grow and passivate, so the tube could have seen a higher temperature than the color indicates but only for a brief time. I never really thought about how the discoloration would differ in different mediums (I just assumed it would be the same for any oxygen-bearing medium). Do you have a reference on the subject?

The oxygen-bearing medium is what XL83NL is referring to, according to me.
If it happened in an inert atmosphere, there would be no discoloration, no matter how high the temperature went.

There's no context here.
What are the materials?
What is the environment?
Is the discolouration due to corrosion or deposits?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."