set_character_color_shading

Sets the intensity of the color slot’s shading, with 1 being normal shading. Setting it to 0 will make it completely flat, reminiscent of the Early Access palettes in the base game. (Note that you aren’t restricted to just 1 or 0.)

//This code sets the first color slot to have completely flat shading if they're set to a specific palette.
if get_player_color(player) == 12 {
set_character_color_shading( 0, 0 );
}