[Amps] Question about pre pre pre drivers

[Steve Thompson]

A friend reaised a question and I wondered if folks here might know the 
answer.

Here's a picture of a 5B/254M (a miniature 807) 
http://www.tubecollector.org/5b254m.htm - there's a lick of paint (or 
something) on the anode. It's not for decoration so what's it for? Is it 
something that changes colour as a telltale for overheating, or an 
indicator of being close to maximum temperature?

It's not unique to this particular valve/tube but seems to be more 
common in 'quality' brands.

Steve