I'm sure this has been brought up before but I'm curious now that I'm building a non-5.56 upper.

How do you (if you do) make it obvious which upper is which if they're similar? Both uppers will be similar. Especially to someone who's not such an ammosexual.
What about mags? I take friends shooting and I want to make it clear which is which.

I think I'm going to do all FDE mags are .300blk and black will be 5.56. Also considering having the .300blk upper cerakoted FDE.