An extremely reliable opinion I consulted tonight casts great doubt on them ever being anything but grey. I suspect that if any were ever red, it didn't last long. In those days, separate liveries eg the Asahi, Tara or Ammonia were unusual and private owner / reserved operator stuff generally carried the standard livery, ie grey.
The photos show above both suggest grey, though with the top one it's hard to tell. While the chassis on it looks darker, maybe black, so does the wagon hogs to the left - and it would certainly be grey....the picture exposure is the wild card there.