Red also works great if you want to play a prank (not that I ever have)
Either a prank or security feature I used:
Weld a hex nut on a piece of threaded rod. Cut close to the un-welded side. Drill through with the correct size tap drill. Re-tap the nut and the remaining part of the weld.
Use it and it looks like the nut is welded on. Easily removed though.
Works great for 1/2" and larger, smaller sizes tend to push the weld out instead of cutting into them.