The diffraction grating works on the principle on wave interference. If the light waves interact constructively we see a bright spot, and if they interact destructively we see a dark spot.
What happens when you shine light through a diffraction grating?
If it's white light, you get a rainbow
If it's a red or a green laser, you get a red or green diffraction pattern
If it's a multispectral laser like the Argon laser in Chamberlin, you get the visible colors that make up the beam