The need for a long service life
The electrode format of the EF91 and similar valves.
Inside the EF91 (and similar valves) the anode runs round only part of the cathode/grid structure, and so a significant number of electrons pass through the grids and the gaps in the anode, and into the glass envelope. It was found that this gradually caused gas to be released from the glass, and over the extended lifetime of high reliability valves required by, for example the GPO, it was sufficient to impair the efficiency of the valve and to shorten its life. Therefore, the centre part of the glass envelope was coated (by vacuum deposition) with a chemical. This kept the electrons from the glass and thus reduced the gas released, and thus lengthened the working life. The actual colour of the coating varied with the temperature of the firing, and has no significance.
A Mullard EF91.
I particularly like the bluish colour of the glass around the anodes in the Mullard valve.
Some manufacturers of EF91's use a different technique to protect the glass from these stray electrons. For example, lve seen some M-OV CV138s with what appears to be a metallised internal coating.
You can find many articles in the amateur press using the EF91. For example, Practical Wireless published The Versatile EF91 by K V R Bowerman in its February 1964 issue. This article described a 5-valve switched MW/LW superhet and a 4-valve push-pull audio amplifier, both using 100% EF91 line-ups. Another all EF91 project was 'A Simple 3-Valver by T M Bush in the December 1962 issue of Practical Wireless, as was A TRF Communications Receiver by R H Wright in the September/October 1959 issue.