While white engines from the factory were the "norm" in Peterbilt trucks until about 1986 or so, it was not uncommon to find trucks from the factory with OEM colored engines. In this case, painting your Detroit Diesel Alpine Green is totally fine.
Great job on detailing your Aries K die-cast. I had one of these when I was in high school. It was a two door, dark metallic gray, double red pinstripes, and a red interior. I think it was a 1984. Anyway, enough reminiscing. Very well done.