If you mean via CSP, you can do it via ext_config by using this
[SHADER_REPLACEMENT_...]
MATERIALS = NAME-OF-MATERIAL(S)-OF THE-GRASS (divided with commas
PROP_...=ksAmbient, 0.25
PROP_...=ksDiffuse, 0.25
*INSTEAD OF MATERIALS you can use also the MESH name
MESHES = NAME-OF-MESH(ES)-OF-THE-GRASS (divided with commas)
Then tweak the values of the ksAmbient and ksDiffuse, for the 99% of tracks values between 0.20-0.30 are more than ok to have a right colors