A lens filter that absorbs some colors and allows others to pass through to change the colour of your image.

Depending on the pigmentation of the optical filter, the contrast in the image might be increased or reduced, and some color tones will be enhanced.

So an example of this could be with a yellow filter which absorbs all of the colour spectrum except yellow. This creates more contrast between blue and yellow in the scene you’re shooting which is why it is popular with black and white film photography – it adds more contrast.