What is the principle that light is colored when it passes through the gap between two glass plates?

When you pinch the surface of two glass plates with your fingers, you can see color stripes, which are formed by the superposition of reflected light from the upper and lower surfaces of the air film, and it is an interference phenomenon. When you look at an incandescent lamp through two pencil slits, you will also see colored stripes, which is a diffraction phenomenon.

So the answer is: interference diffraction.