That color is simply known as "aluminum", but in the name of all that's holy, DO NOT REPAINT OR TOUCH THE PAINT ON THAT CAR! That little 654 is in VERY good condition, for pity's sake, don't mess with the finish or you'll destroy it.
Replacing the missing trim is fine and should be easy enough to do. I've seen mixed nickel and brass trim on these cars before, I have an orange one somewhere packed away that has the same thing. These were toys and Lionel was just using up whatever they had available at the moment.
If you get repro trim for this car and want it to match and look old very quickly, here's a little trick. Boil a couple of eggs, crumble the yokes up in a ziplock bag, and place the brass or nickel parts in the bag and leave them there for a couple of days. Due to the extremely high sulphur content, in less than a week, that brand-new stuff will look like it's a hundred years old, then you can clean them up to look like the pieces on the tank car already. It's a very old trick, a LOT of antique auction folks do this with repro items - works equally well with nickel, brass, copper, silver, gold, or pretty much anything that's capable of tarnishing. Good luck!