use material :

// Just an example
Code:

---

material myMtl
{
        ambient_red = 50 ;
	ambient_green = 50 ;
	ambient_blue = 50 ;
	diffuse_red = 0 ;
	diffuse_green = 0 ;
	diffuse_blue = 0 ;
	specular_red = 0 ;
	specular_green = 0 ;
	specular_blue = 0 ;
	emissive_red = 40 ;
	emissive_green = 40 ;
	emissive_blue = 40 ;
	power = 0 ;
}

action myEntity
{
    my.material = myMtl;
}

---

But not sure if it works with your GS version.