You can take your choice, any color (except maybe black) can be hard to get "right." One of the hardest is the "imitation aluminum" paint used on CB&Q freight F-units.
It can look anywhere from gray to white and even beige under certain conditions. Model train companies in olden days used silver, which is nowhere near the way the units looked.
Update: Adding the photo I usually show when this type of subject comes up. Same loco, different lighting. The manufacturer matched the EMD color chip.
Rusty