What colour button down shirt should I wear under a black sweater vest?

Black goes with anything, remember?

Black is a neutral, and goes with every color you can think of. The trick is to coordinate it with the pants and shoes you’ll be wearing with it (both style and color).

I wouldn’t pair it with navy blue or a very dark purple, green, or brown, but just about anything else would look fine.

Black does go with a lot, so now you just have to make sure it coordinates with the pants and shoes.

Off the top of my head, I’m going to say a regular blue or lighter blue with pants that aren’t too light or too dark. This is very vague, but you get the point. And wear black dress shoes. I recommend Clarks.

Blue or green.

A bleached white shirt would really stand out.

Dark green. It’s my default answer to any color question.
(Just don’t wear it anywhere near me, or you might find yourself without a shirt.)

If you want to look like Halloween: Red, purple or yellow.

If you want to look classic: white, gray.

White.