When did the D&H change from tuscan colored boxcars to the yellow and blue scheme?
The first yellow D&H boxcars with blue lettering were 50' PS-1's delivered in 1964. I remember my surprise when I saw a nearly brand new one in Oberlin, Ohio.
However, D&H 40' boxcars remained either boxcar red or oxide red. The only one I'm aware of that was yellow was specially painted for the railroad's Sesquicentennial exhibit train.
The definitive source for information of this kind is The D&H Color Guide to Passenger and Freight Equipment from Morning Sun Books.