Ian, it kinda depends on the PBX. Some use one pair and some use more than that. You will need to check and see how many the system that you are working with uses.
You should use the same colors for a pair such as blue, orange, green, brown.
I use a CAT5 routinely to run say a digital extension, fax and credit card machine. It's best to run a dedicated line, but not always somtheing that you want to do.

Paul