Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: GRASS_MAT_PREVIEW
GRASS_MAT_PREVIEWParameter: MATERIAL_PAGE_PROPERTIES
MATERIAL_PAGE_PROPERTIESParameter: Color
GRASS_COLORParameter: Color Texture
GRASS_COLOR_VAR_SHDParameter: Mix Strength
GRASS_INHERITCOLParameter: Blade Length
GRASS_LENGTHParameter: Blade Width
GRASS_WIDTHParameter: Density
GRASS_DENSITYParameter: Density Texture
GRASS_DENSITY_SHDParameter: Segments
GRASS_SEGMENTSParameter: Crinkle
GRASS_STYLE_1Parameter: Bend
GRASS_STYLE_2Parameter: Wetness
GRASS_SPECULAR