Colour filters work by filtering out specific wavelengths, allowing only certain colours to pass through. Therefore, colour filters absorb all colours except for the filter’s colour, which is transmitted.
For example, if we pass white light through a blue filter:
White light contains all different wavelengths of visible light. When white light passes through a blue filter, only the blue light is transmitted, while other colours are absorbed. This results in blue light emerging from the other side and reflecting into our eyes.
An object will appear black if the filter absorbs all the wavelengths of visible light. For example, when green light passes through a blue filter:
If green light passes through a blue filter, no colour is transmitted, so the light will appear black.