The orange colour came in with APTIS. Previously the normal colour was red, at least for the most common ticketing system, the NCR51 and also for INTIS. As someone has already said, orange was very much in vogue in the mid 1980s.
As for different coloured APTIS blanks, there has been a vast array over the years (over 200 different types) covering various seasons, Travelcards, Railcards, etc. A few years ago there was a move to common ticket stock and the vast majority of tickets went over to the standard orange stock. In order to achieve this the orange stock was altered so it was blank apart from the security background. Previously various headings etc were preprinted onto the ticket. The aim was to reduce costs and speed up transactions. These days only Railcards and some seasons are printed onto bespoke ticket stock.