Yellow was for business. The giant white book was for people.
I mostly remember them being combined into one big yellow book, but separated by page color (yellow for business, white for personal).
It’s interesting how universal the use of colour was. I grew up in a smaller province most people don’t know (or at least can’t spell). The phone books were made by a crown corp that was pretty much just for the province. Yet, same colour schemes. Outside of the book was yellow. White pages for people, yellow for businesses.