Red and orange will yield a "darker " green leaf, yellow probably a slightly "lighter" green leaf since some yellow filters are yellow-green, green would give you a "lighter" green leaf . Check to see what yours is. The red and orange will probably have the opposite effect on the brown leaf. Try all as suggested as well as no filter which may turn out the best. Generally I would use a blue filter if I wanted to accentuate fog in a landscape under foggy conditions. Most of the time red, orange and yellow would be used to "darken" the blue of the sky and bring out the contrast of clouds against the sky. You might also consider orange to increase contrast in a landscape and green to lighten up green in a landscape. Some will use filters such as red to lighten a ruddy complexion or for a rather white skin for Caucasian subjects.

Remember that in B&W you are dealing with values and the brown and green leaf could actually have the same value and with no filter come out in similar shades of gray so using a filter might be necessary for your purpose.