Paint to protect schedule 40 black steel pipe running outside above ground
Given that you're painting metal, I would avoid latex paint (which is nearly every can of paint in the home-improvement store).  

The #1 issue is paint prep, especially if there's rust.   Rust is very difficult to stop if it's underway under paint.  Any way you can remove it; sandblasting is not excessive.**   Paints that claim you can paint over rust are lying, sure, you can do it, but the rust will punch through in short order. 

Then a good anti-rust primer (I've been surprised by the performance of plain old Rustoleum 7769 Rusty-Metal Primer, especially if the rust removal is less than perfect).   Then a quality oil paint.  

Generally, colors will not hold up in hardware-store-priced paints, because mineral pigments like lead chromate (shudder) are illegal, and organic pigments that hold up are prohibitive ***.  Fortunately carbon black is a mineral pigment not toxic to humans, so you can get cheap good black paint. And that's a fine color for a "black pipe" gas pipe. 



** In fact, in extensive testing by NASA (who owns more steel structures in corrosive environments than anyone in the world), sandblasting is proven to be the only method that really works, with significant performance gains over needle-scaling and heavy sanding/grinding.

*** To give you an idea, I normally use top quality LPU over epoxy primer. I'm used to paying $160/gal. for black or white (but mind you it covers 1000 square feet).  I ordered some burgundy, and it was $295/gal. because of the pigments.  I do not recommend using LPU over epoxy because the cost of initially getting set up in the primers, converters and other chemicals is north of $400, a bit extreme for a small job.