I know that #50 was the assigned switcher at Helena, MT in the 1950's. There is a picture of it in Helena on page 150 of the book The Montana Central Railway by Bill & Jan Taylor. There may also be photos in GNRHS Reference Sheet 239 on the Montana Central, but I no longer have a copy to verify it. There is a picture of #51 on page 129 of Railroad History Bulletin 143 (R&LHS). Both pictures, while black and white, clearly show the units in the two-tone orange & green scheme. Given the build date of 1940 I am sure they were delivered painted black.
GN purchased the two 44-tonners, numbers 5200 and 5201, built by GE (S/N's 12910 and 12911) in September 1940. They were re-numbered 50 and 51 in 1943. These switchers never received a class designation from GN. According to the all-time GN locomotive roster in Railroad History Bulletin 143 (R&LHS), #50 was sold in late 1951 and #51 in early 1948. The picture in the Taylor book states it is 1955. So, at least one of these references is wrong... I hope this info helps...
