[Dekatron]

The original 2N2926 were sold with different gains, hence their colour coding, and the price varied with the colour code. So it was just a way of testing the transistors during manufacture and then selling them according to what hFE they had I guess.

To get the circuit to work properly AVO had to select hFE properly, therefore they specified what colour each transistor should have. There is one change note where they specified that VT3 could be any except brown, so they were aware of the problems with incorrect hFE in some positions.