Description of the effects of Hoya X(0) yellow-green (1.5 stops in daylight) and X(1) green filters (2-stops in daylight) are here.
Additional notes from Ira Tiffen on the Tiffen Filters site:
“A green filter, such as Wratten #11, can be used to lighten green foliage, to show more detail. It may also be used to provide more pleasing skin tones outdoors, especially against blue sky.
Any filter used for the above purposes will have a greater effect if slightly underexposed. Its function depends on absorbing light of its complementary colors to increase the proportion of light of colors similar to itself. Exposure compensation is often needed to allow proper image density, but the relative difference is reduced by the addition of light at the absorbed wavelengths through additional exposure.”