I've picked up a few vintage Chinese bikes, and have been curious about the plates. This is what I've uncovered ...

Prior to the current Yellow (= & > 50cc) and Blue (< 50 cc) plates, of the 京A 京B variety, the plates came in:

1) White with red letters/numbers (< 50 cc); and,
2) Green with white letters/numbers (= & greater than 50 cc)

They also came in:

1) 北京 01 (the equivalent of the current 京A plate); and,
2) 北京 02 (the equivalent of the current 京B plate).

I am not 100% sure when the switchover was made, but I am guessing early 1990s. That seems to be the consensus on motorfans. Maybe someone else knows?

Here's a picture table (sorry, couldn't find a good example for a green 02)

Vintage Beijing Plates.jpg

Other vintage plate info is welcome (e.g., from other cities).