In the current catalog one can see a big difference in the Blue Comet colors between the brass trim and the nickel trim. It shows up in the O gauge set and the 390E. The catalog shows nickel trim Blue Comet cars and yet in the description it says Brass trim among features. Maybe they will make both. Either way both trims are readily available.
I ordered the 390E in brass trim because this was the first engine to pull the Blue Comet with the big 400 series cars. I thought they would offer brass trim because of this. Good thing I already have a set of brass trim cars. I suspect many blue Comet owners will purchase a 390E as an alternative engine to the 400E. I also suspect the nickel trim 390E will match nickel trim cars in color. It would be a nice way to put modern electronics into a nickel Lionel Classics set.
Generally speaking brass trim represents a time span that ends in the 30's. After this year which I do not remember Lionel switched to nickel trim. As far as I know the Blue Comet is the only set with a color change. That being said the 1766 cars are 2 completely different colors with nickel or brass trim.