Let's talk about how to match a black slim suit. T-shirt with round neck or V-neck? What color is good?

In view of your thin figure, I personally suggest that a T-shirt with a round neck will make your face look long, while a T-shirt with a V-neck with a suit will make people with a slightly wider face or a slightly fatter face look better, so that the gap between the suit and the neck is not so big. If paired with a V-neck, it will give people a slim feeling from the front or the side, not as full as a round neck, and the colors are military green, burgundy, white and gray.