Doors

Doors are contraptions made of metal, and can only be opened if you hit the blue button (or both if there are two. The door will not open unless both are hit.).

In the introductory stage, there are blue arrows pointing to the buttons.

The buttons are reused in the Local Multiplayer Modes for one glass pane.

Buttons
Each door has 0, 1 or 2 buttons. Only hitting all of them will open the door. The buttons are also used in the glass pane with buttons (exclusive to Local Multiplayer, specifically Versus Mode).

Outline
In the Tunnel to Endless Mode (first opening in all versions, and second opening as well before version 1.2.0), as well as the Disappearing Room Glitch, and opened-up doors, the doors will have an outline. It is transparent - despite it existing, balls will go through it.

Trivia

 * Smash Hit - Official Trailer starts and ends with a door opening.