Ilford has more information on the effects the filters produce here.
Those of you old enough to remember shooting black-and-white film might also remember that purists would then add a filter to the lens to accentuate something in the scene. Red adds the most overall contrast, yellow is good for skin tones (and foliage), orange somewhere between red and yellow, and green mainly for foliage. Ilford has more information on the effects the filters produce here. However, I’m not looking to carry more filters around with me. And as it turns out, I don’t need to. In Nikon’s Picture Control settlings (and some non-Nikon cameras my clients have used) I can select one of those four filter effects when I choose Monochrome. So as a default now, I leave my Monochrome set to +2 Contrast and Red filter, and can quickly switch between that and color.