GMS 2 Lighting with surfaces: can't change color of lights

I'm setting up a basic lighting system in my game. There are two objects involved; obj_lighting, which creates the surface and handles all the drawing, and obj_light_source, which I place in the room at points where I want there to be a light source.

I've put a variable in creation code of the obj_light_source called tint_color that is meant to allow for changing their color. I put some lights in the room, opened their creation code, and set the tint_color variable to c_red. In game, however, they are all being drawn as white. I tried upping the alpha on the light sources all the way to 1, but that didn't seem to help.

Relevant code (being run in obj_lighting draw event):
Code:
/// draw surface, blend, draw lights
if !surface_exists(light_surface)
    {
    light_surface = surface_create(width,height);
    }
else
    {
    draw_surface(light_surface,0,0);
    surface_set_target(light_surface);
    draw_set_alpha(dark);
    draw_set_color(c_black);
    draw_rectangle(0,0,width,height,false);
    if tint == true
        {
        draw_set_alpha(tint_amount);
        draw_set_color(tint_color);
        draw_rectangle(0,0,width,height,false);
        }
    draw_set_color(c_white);
    if spotlight == true //spotlight on player
        {
        gpu_set_blendmode(bm_subtract);   
        draw_set_alpha(.5);
        draw_sprite(spr_light_64,image_index,obj_player.x,obj_player.y-6);
        gpu_set_blendmode(bm_normal);
        }
    //draw light sources
    if instance_exists(obj_light_source)
        {
        with obj_light_source
            {
            gpu_set_blendmode(bm_subtract);
            draw_sprite_ext(sprite_index, image_index, x, y, xsize, ysize, 0, tint_color, alpha);
            gpu_set_blendmode(bm_normal);
            }
        }
The obj_light_source objects do not have a draw event.
Some of the above code is likely irrelevant (like spotlight on player and tinting the surface) but I left it all in just in case.

Thanks!
 
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