Pick a color that has a light and dark contrast with the background. The background is usually the color of the house. This will make the number stand out so it can be viewed from the street. If your house has a dark color, you need a number in a light color. If your house has a light color, get a dark color number. Please use caution using brass or bronze numbers as they are very difficult to see on many backgrounds.
The number needs to be a light and dark contrast with the mailbox color. If you go with stickers, the ones with black numbers on a white reflective background are excellent. Another option is to mount a sign above or below the mailbox. Don't get a color similar to the mailbox's color.