Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Color 1
STARSHADER_COLOR1Parameter: Color 2
STARSHADER_COLOR2Parameter: Streaks
STARSHADER_STREAKSParameter: Inner Radius
STARSHADER_INNERRADIUSParameter: Outer Radius
STARSHADER_OUTERRADIUSParameter: Density
STARSHADER_DENSITY