Sahad Vasanji (Aizenberg Lab) explaining how you get colors from completely transparent materials. It's all because different materials have different refractive indices. When light encounters a boundary between two materials with different refractive index (even if they're transparent!) it will get refracted and reflected. That's why a pencil looks bent in water. The reflected light can either add (constructive interference) or subtract (destructive interference), depending on the wavelength of the light. Whichever wavelength adds up is the color you see.
