The observations used on the Laurentian were all New York Central cars that were unpainted stainless steel. They were taken off the train sometime after 1962, when I saw one in Whitehall, and 1967. At least two NYC stainless observations went to the Kansas City Southern; several went to the National Railways of Mexico. Model Railroader ran an article, I think by Andy Sperandeo, about kitbashing one of the KCS (and thus also NYC) observations in HO. The observation cars assigned to the Laurentian were numbered in the 60's -- that's right, just two digits each.
The blue, gray and yellow colors were first applied to D&H passenger cars in 1967, when half-fluted cars were bought from the Denver & Rio Grande Western. There were no observations in that group.
The D&H did, however, have some heavyweight business cars that had open rear observation platforms. At least two of them w ere eventually painted blue, yellow and silver and/or gray. Athearn's standard (heavyweight) observation car could be used as a somewhat shortened (in length) approximation of the one of the two business cars that had a regular clerestory roof, not an arched one.
Probably the best reference book for this sort of question is the D&H Color Guide to Freight and Passenger Equipment, prepared by four members of the Bridge Line Historical Society and published by Morning Sun Books. There is also an excellent book on D&H passenger service in its final decade by Doug Lezette; it's almost entirely devoted to the period from 1967 on; and, unfortunately, it's out of print.
