I presume that the rostering of sets involves splitting 8 car into 2x4-car formations at some point in the day, and re-combining 4-car into 8-car formations at a later stage, possibly in a different location. Also individual 4-car units may have different maintenance schedules or may develop faults, which involve swapping some units.
So I'm fairly sure it's more complicated than you seem to think. The last thing on the minds of IE personnel must be the matching of liveries. There are time when IE needs a break!