Thin wall as a gel
Make a Principld BDSF, apply to a plane, add a texture, set transmission to 1, and turn on thin wall. And crucially: Roughness to 0
Voila. Colour transmitting gel. Shine a light through it'll cast through volumetric and onto surfaces.
Any roughness value over 1 turns this off.
You can apply bump or displacement but it will not display any diffraction, so if you need diffraction, use a plane with normal glass, turn shadows off, and just stack them














