it's a combination of factors:
Halfway through the previous franchise, with some of the fleet still in previous operators colours, National Express rebranded from
'one' (blue, green doors, rainbow stripes) to
National Express East Anglia - white, with dark blue doors, silver ends to the carriages and their "dot to dot" silver diagional stripes. This was going to be the livery for all National Express operations- even the coaches. When National Express East Coast collapsed the following year the unified branding was dropped, with further repaints simply being into
plain white with dark blue doors. The franchise was supposed to end in 2010, but the franchising review saw it limp on a further two years.
Greater Anglia took over on what was supposed to be 16 month franchise. This will in the end be a four and half year franchise. The franchise was awarded as a "caretaker", so included few commitments to invest. the repainting was continued as required, with red doors instead of blue.
They've now moved on to a modified version of this, with a light grey band low down and a darker grey window band. This seems to be getting applied a bit quicker, and even (I think?) being applied to units in all-over white. But don't hold out hope of the fleet being in a single livery by the end of the franchise! Some of the DMUs are still in the Anglia livery they were originally delivered in in 2002.