You may well be on to something there, Nick.
I have always assumed there was a mix of liveries, based on the livery change dates. And I remember seeing it mentioned by others who model the period. But now that you mention it, I don't actually recall *seeing* all that many trains with mixed liveries in photos. Mixed stock, certainly, but mixed liveries - not so much maybe.
That said, I assume the all-over colours (brown, lake, and maybe even khaki) might look fairly similar in a b/w photo, unless you were looking for differences. In any case, a good excuse to have another browse through the books.