Even though black and white film only displays greyscale images, it is panchromatic, which means it is sensitive to all visible (and some invisible) wavelengths of light. Therefore, in order to preferentially let in certain wavelengths, at the exclusion of others, you can use colored filters in front of the lens. They’re used to create certain effects, like darkening the sky or lightening foliage to make flowers stand out. There’s a really good blog post on the subject here, which really made it all click for me. As a musician, I think of colored filters like wide-Q bandpass filters for audio. So I can throw on a lowpass filter on a bass track and make it really boomy and low endy and cut out all the highs. Similarly, I can throw on a red filter (that’s the lowest frequency of visible light) and make all near-red objects lighter and all higher wavelengths progressively darker.

What was my point in posting this? Oh yeah, I wanted to give you my own examples, below. It’s just a random NYC street, at shutter priority (I really should have matched the aperture so that DoF wouldn’t have been so much of a factor, but it’s too late now).