Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Index
GV_VERTEXCOLORMAP_INPUT_INDEXParameter: Color
GV_VERTEXCOLORMAP_INPUT_VALUE_COLORParameter: Alpha
GV_VERTEXCOLORMAP_INPUT_VALUE_ALPHAParameter: Color
GV_VERTEXCOLORMAP_OUTPUT_VALUE_COLORParameter: Alpha
GV_VERTEXCOLORMAP_OUTPUT_VALUE_ALPHAParameter: Count
GV_VERTEXCOLORMAP_OUTPUT_COUNT