Vertical Integration
Forgive me if wrong, but I think that we may be missing the point. The UK charter carriers are part of vertically integrated businesses (less true I believe of Monarch) - The airline livery is not a discrete item, but part of the overall group image and packaging - therefore it is the group image tht determines the colour scheme applied.
Customers by and large don't know (or usually care) who they fly with, but do view the flight as being part of the product purchased i.e. a holiday/vacation and are reassured when they see the corporate logo and colour scheme carried through the experience.
Those carriers (such as Monarch who operate scheduled services) that have a lesser allegiance to a specific Tour Operator will consequently need to have a more distinctive livery, and it may be more important to their business results.
This is what I have always been told by the Marketing types in our org - doubtless I stand to be corrected, and knowing PPrune, probably will be so...