The thumb rule of matching belts with suits is: the color and finish of the belt should match the color and finish of shoes.
This is really simple. Black shoes, black belt. Brown shoes, brown belt.
Another thing to remember: The belt buckle finish should match the other accessories that you are carrying - if you have a gold accented watch and a matching tie-pin, you will need to chose a belt with a golden buckle.
For the sartorially adventurous gentlemen that dare to wear pastel color suits with canvas shoes, the choice expands to include fabric belts in matching colors.
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