This is copied from a PC devoted website (
www.smellycat.com) Hope it helps.
The PC emblem, commonly known as the "mating worms", was originally applied to locomotives in white. All freight cars, except open hoppers, received a black PC emblem over their gray (covered hoppers) or green (everything else) paint. By April 1968, though, the black logo was replaced by the all-white log on all freight cars, except for on gray covered hoppers.
In May 1968, PC experimented with the "red P" logo, consisting of a red "P" and white "C". This version of the emblem was applied to approximately 136 locomotives and some freight cars during the April-June 1968 period. This was discontinued, probably due to the "red P" accentuating the internal strife between the PRR and NYC sides of the company. PC then returned to the all-white logo.
During October 1968, a group of 17 new EMD GP40 locomotives were received with the "Orange C" emblem, featuring a white "P" and an orange "C". This emblem was an experiment which was not repeated, and the company kept the all-white PC emblem until its demise. (Interestingly, though, PC 5585, a former New Haven RS3, was painted with an "orange C" emblem in April 1969, long after the experiment ended.)
According to Chuck French, one possible explanation for the "Orange C" emblem was to make the New Haven feel welcome as a part of Penn Central. The "Orange C" locomotives were delivered in October 1968, a few months before the New Haven was taken over. At the same time, PC was basically forced into merging the New Haven by the Interstate Commerce Commission, so PC management may not have been inclined to change the company emblem on locomotives just for the New Haven people. However, it is the best (and really, the only) explanation that I have heard for the "Orange C" emblem to date.
Sources: Penn Central Power by Robert Yanosey, Penn Central Color Guide to Freight and Passenger Equipment by James Kinkaid, and Chuck French.
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