I think we've been over this - and I can't remember if this is an occasion where Yeadons made an oversight, or if it was so 'unofficial' that it wouldn't/didn't count but... Gorton did double white on an A5 just because it could - it's basically a GC livery - as a bit of a, er, 'do one' to the LNER.
It's a paint shop peculiarity, basically, but it is right (as in it happened, rather than that it was necessarily authorised...). From memory, the last time I dug right down into it it really was a two fingered salute as by the time they did it they'd painted a lot of other locomotives properly.... so they don't have the excuse of 'using up paint' or 'misreading the instructions' that they might have had right on the dot of the grouping.