I think Deere used some shade of green and yellow on the Dain tractor, as well as some red. Earlier plows used some yellow, red, and black as well, sort of like the early Van Brunt wood box drills. Mansur planters were red and cream. But...I think the tractor colors became green and yellow because that was the color combination used on the Waterloo Boy, the popular line that they bought in 1918. Now, I wonder, since the time frame of introducing the Dain tractor to the public was right about the same time or a few months later, that the Waterloo acquisition came about that maybe they chose to paint the Dain green and yellow, with a little red, after the fact, to ease it into acceptance.
Others say that the green and yellow combination was decided on earlier for implements, and long before the tractor experiments/development/acquisition, sometime from 1890 to 1910. The truth seems to be that not even Deere & Company knows. Guess it was lost in the board or committee discussions along the way.
A little bit of color discussion can be found in the "John Deere's Company" by Broehl.