I usually just use the grey slider set at 0.2 or 0.4 or so, but it depends on the species of grass and the effect you're after. Maybe a yellow-green color would be better. I also have some texture (grey scale) loaded for very close-up grass. Interesting to see the diferences in color when you look at fields of grass with light behind you (bluish because of reflection), and againts the light (very warm yellowish green).
Also, I sometimes find the translucency too much for grass in shadows (they kind of lighten up due to GI), as opposed to grass in sunlight.
What I also do is reduce opacity to 0.6-0.8 for less dark shadows between blades.