They Should Match
Your air vents should match your wall color. Otherwise, they will become focal points in the room because of the color contrast. Most air vents unscrew easily, and are small enough to paint quickly.
According to Just Home Decor, your air vent can be a decorative accent in your home. You can get an air vent in a variety of designs that you can incorporate into the decor of your room. You can purchase an vent with an abstract design, or one made of wood to match a rustic room.
Painting your air vents to match your wall color makes them blend in with the wall, leaving your furniture or artwork as the focal point of the room. This is the best design decision. However, you can also purchase a decorative air vent that matches the design aesthetic of the room.