I have worked on thousands of old bikes and had more than my fair share of Pugs pass through my hands and can only recall seeing one yellow one of that mid seventies vintage... the FFS system was used in the early 80's and quickly abandoned.

Green and orange are the most common colours I have seen here, purple frames are a little rarer, and they made this lovely light coppery brown that I have only seen a few times.

I think that a lot of the appeal these old Pugs had was their vibrant colours that stood in stark contrast to the more sedate colours of the competition.