You're probably right... all such photos I've seen (like this one http://railpictures.net/viewphoto.php?id=44543 ) are from the 70s. But why do only the 50s and 60s matter?I still haven't seen any evidence that the Santa Fe intermixed red & silver F units with blue & yellow units or Gp 7's or 9's during the 1950's-1960's.
This ATSF argument is pretty pointless.
EDIT: Red and silver Fs definitely ran on freights in the 1960s:
http://www.railpictures.net/viewphoto.php?id=132242
http://www.railpictures.net/viewphoto.php?id=55157
http://www.railpictures.net/viewphoto.php?id=55156
But here's the clincher: http://www.railpictures.net/viewphoto.php?id=49151 Billboard GP7 and warbonnet F7B on a passenger in 1968. This was in the time when the cowls were new, but still...