Two interrelated questions:

My stationary air compressor will be hard wired and runs on STRAIGHT 240V-ie two hots only. Since the two hots are connected to a double pole breaker, can both wires be the same color or need to be different(red & black)?

Same question for my welder, though this will still use a plug/outlet. The outlet is a NEMA 6-50R. Thanks...

You can use 2 blacks, 2 reds, black and red, and see no reason you couldn't use 2 blues, yellows, browns, or any mix of the above colors, anything but White, Gray, Green or Bare.

Strat,

That's what I kind of logically figured-though logic isn't always correct. Thanks.

For straight 240V you can pull 2 conductor romex and use black and white. Use a magic marker and color the white at the breaker and outlet to black. This is similar to what you do with a switch loop.

ma0641,

Good advice & romex would be easy, but my area requires conduit. Even though I'm not pulling a permit, I like stay in local code. Besides, I paid for these conduit benders and I'm going to use them :)...

Since you will be in conduit, use a red and a black. That way, you know what breaker the wire is connected to.