A lot of this can be down to practicalities. A loco is in for repainting, someone's there to do the job but there's not enough of the correct colour because someone else has forgotten to reorder. Rather than delaying / putting off the job, paint it with anything acceptable on hands because it's desperately wanted out in traffic again. We see oddball liveries to this day for pragmatic reasons, the public won't care what colour the loco pulling their train is.