Should you wear a bow tie?
If the invitation says black tie, then wear a bow. Save the necktie for the invitations that read - festive or formal attire.
Must it be black?
Yes. The choices of fabric textures and shapes give you the variety of options: shiny, matte, cotton, silk, pointed, butterfly shape...
Can you wear a pre-tied bow?
We would kindly ask -- please don't. The whole fun of wearing a bow-tie is in how you tie it. And then untying it at the end of the night/day. Everyone knots theirs differently, which is ultimately how you express yourself and radiate confidence.