Hmm. The D7 kit I had back in 1971 was in white plastic.
It's possible that was an unusual pigment run. The one I had back around that period was gray, as was another I saw a kid playing with on the playground in third grade. The unbuilt one I currently have in the basement is also molded in gray. And this one being sold on E-Bay
shows gray parts still on the sprue and in their bag.
By the way, I found an interesting article on the original model
that confirms it was painted in purple and green. I guess you really can't argue with RGB values.
Those colors are not "green" and "purple" though.
They're "grey with a hint of green" or "grey with a hint of purple."
When I mixed my own version of those colors, it was primarily a standard grey color, but I added a small amount of forest green to one and a a small amount of blue and red (equal amounts) in the other. The base grey color was the same in both cases. I recall using light ghost grey. I then darkened the topside color slightly using battleship grey (which has quite a bit of green in it naturally). The end result was something I was, and still am, very happy with. Most folks who look at it don't immediately notice the different shades of color, and assume it's just a trick of the light (which, I'm convinced, is the whole point of why it was painted that way in the first place).