Colour is an essential component in the process of crafting a fine Black and White image or print. In Black and White film photography different coloured filters are attached to the front of the lens to increase contrast and distinguish colour from tonality by filtering certain colours from the spectrum. For example, with Black and White film a red filter is often used to deepen a blue sky and make white clouds pop. This happens because the red filter lets the red wavelengths pass through to the film but absorbs or filters the rest of the colour. Again, using Black and White film, if I was photographing a red and green ball that were both the same tone, or contained the same amount of colour, the resulting Black and White image would show two identical balls, however if I added a red filter to my lens the result would be quite different with the red ball appearing darker and the green lighter, therefore using the colour to achieve tonal separation in the Black and White image.
Essentially, Black and White film can “generally speaking” only distinguish between tones not colour, so by using a filter we can control what “coloured” light is transmitted through to the film and what is absorbed, and therefore providing some control over tonality. This is however something that can be a little hit and miss or a little less predictable for someone who does not have the experience.