I paint my middle rails rust brown same as the outer rails. Just my personal preference over black. The Gargraves Flextrack being used on my current small round the room layout is all recycled from larger layouts dismantled in '08 and '09. Some has Phantom Rail and some plain. Switches are Ross/Curtis.
I tried several shades for rail-rust painting and after painting several test spots over the real rust on some real rails north of town, I settled on Tamiya XF-64 Red Brown.
If you have a lot of track the inexpensive way is to buy one small jar of your selected acrylic rust color and get it computer-matched for a small $5 sample can at Ace, or Home Depot.