Collar type, color, and style are three things. Collar types include collars (high collars, lapels, small stand collars, etc.) and collarless (round collars and triangle collars). They should be matched according to the individual's face shape. Generally speaking, business casual styles are collared, while athleisure styles are collarless. The color should match your skin tone. For people with fair skin, any color will suit you, but for people with dark skin, it shouldn’t be too flashy. There are loose and slim styles. If you are young and confident about your figure, slim fit will be better. Good luck choosing your favorite T-shirt!