As far as SWT goes, initially, both the Mark 1 units and class 455 units gained an Stagecoach orange stripe along with some of the Class 442 units.
The plan was for the white stagecoach livery to apply throughout originally hence the Class 458 units were delivered in white. By the time that the Desiro order was placed, the decision had been made to have a different livery (i.e. Blue) for Outer Suburban Class 450 units. This caused the anomaly that the Class 458 units carried the wrong livery, later corrected when they were reformed into 5 car sets.
The Red livery was the final to appear at the end of 2004 when the Class 455 refurbishment began.
The EMT franchise did not start until 2007 so the base livery set was already well established. This followed the same pattern that First Group had already instigated by having similar corporate liveries for First Great Western, First Capital Connect and First Transpenine franchises.