[johnnyrod]

Yep the "filter factor" tells you how many stops you need to add to the exposure. You can use a lot of different coloured filters with mono, red will give you really dark skies for example, yellow is the de facto for good contrast. A coloured filter also means any aberrations in the lens are negated as you're only focusing a small range of wavelengths, which is much easier for any lens than a whole spectrum.