It must be remembered that many depots didn't keep their sets in fixed formations, therefore when a vehicle was due exam, repairs or works it would be knocked out of the set for relevant work to be done. A spare vehicle would be added to the set.
So when a livery change came about a lot of mixed sets would be running round.
The depot I worked at had blue/green and blue and grey/ green sets the later had blue/white and blue sets, the even blue/white and blue/grey and blue, and we did try to keep units the same livery but it didn't always work.
Al Taylor