The most formal dress shirts are of course white, french cuffed, spread collared and made from a smooth fabric. But when a man wants to dress more casually there are ways to reduce the formality without the usual practice of using more color or a larger pattern. For example, the cotton Chambray shirt in the photo is meant to be worn under a sweater or an odd jacket, with or without a necktie, and has a hidden buttondown collar that adds a touch of carelessness to its look.
Chambray, also called cotton batiste, is a lightweight, tightly woven cloth with a slightly glossy surface due to reflection from white threads in the warp. Neither the cloth nor the color draw any notice.
The shirt's casualness comes from its straight point collar that is worn without collar stays. The points are attached by buttoning tabs under the collar that permit movement while holding it roughly in place. With a necktie, the points are always just slightly askew, without the fussiness of an unbuttoned buttondown.
And that is dress shirt informality.