I sometimes use an orange filter. It will darken the sky more than yellow, less than red, and darkens foliage more than yellow. Adding two air/glass surfaces can reduce contrast slightly due to flare (flare from the subject itself and reflections between the filter and the front element of the lens, which are not affected by a hood, though one should always use a hood for other reasons), but orange generally increases contrast with B&W film, and reduces chromatic aberration by cutting out other wavelengths, so with B&W film, a strong monochromatic filter can actually increase resolution, depending on how well corrected the lens is.