Here are my recommendations for pairing formal shoes with some of the most common suit colors.
Shoes for Blue suits: Being a versatile color, blue can work well with various colors. For your blue suit, you can choose Tan, black, brown or burgundy colored shoes. I would recommend you go with Classic Oxford, Double Monk Strap shoes or Slip-On.
Shoes for Black suits: A pure black suit has only one option and that is black shoes. Anything other than black would surely stand out but in a wrong way. However, you can choose various types of shoes such as Classic Oxford, Double Monk Strap, 4 Eyelet Derby and Formal Brogues.
Shoes for Grey suits: For light to mid-grey colored suits, you can wear light brown, burgundy, and black colored shoes. Lighter shades will convey easy vibes whereas black colored shoes will provide for an enhanced formal look. I would recommend Classic Oxford and Double Monk Strap to go with your grey suit.
Shoes for Olive Green suits: Olive green suits allow you to stand apart from the crowd and make a powerful style statement without appearing to be over-domineering. These are appropriate for both formal and semi-formal occasions. For olive green suits, the best option is Tan shoes. You can choose formal shoes such as Slip-On, Monk, Formal Brogues, or Chelsea Boot.