It's more likely the routes were specified as such by the college.
No, it's because they need repainting and First Bus corporate is the current livery.
I expect that for the Bicton routes in particular, Stagecoach may have put forward a proposal to accept College passes on their public services to connect with the college contract vehicles. First allow (or in any case did at one point) Cornwall College students to travel for a flat fare of £1 on their public services. Prehaps somone more knowledgeable could confirm if this is still the case.
Theres so many different rules come and go, but the last i knew cornwall college gets a child fare upon presentation of a college pass. Rarely used tbh. At the current rate of child fares ,that generally only saves them 20% or so. Thats if the child fare is even set right, lots are blatantly wrong and i think most drivers have given up reporting incorrect fares as the company do not seem concerned.
Some years ago, the Truro College bus pass gave you free travel on other First services. It was of no use to me as I was in Western Greyhound territory except for that college special. I think I just bought my bus pass from the council instead as it was easier/cheaper to get (no queues) but which also didn't give me that "benefit"