It would be fairly easy to put a freight loco in service and run it. Sometimes that happens. But, there are often equipment specifics that make this difficult. Cab signalling, train stop, etc, plus any equipment specifics that the foreign RR employees aren’t trained for - control specifics, brake set-up, how to handle failures, etc.
I’ve always thought it represents the bright yellow line. Kind of like the source of the yellow line that then continues down the side of the trainset. Hence the name “Brightline” for the train service. Plus it is for visibility. That is why the engines are primarily bright yellow.
Unconfirmed report the next 2 Brightline train sets will depart Sacremento Friday May 5th 1200. Since UP has been late for the first 2 any reason to expect these to leave on time?