With this function you can set the background colour for a room (except the current one) and choose whether it should be drawn (true) or not (false).
room_set_background_colour(index, col, show);
Argument Description index The index of the room to set the background colour of. col The new background colour. show Whether to show background colour (true) or not (false).
global.rm = room_add();
room_set_background_colour(global.rm, c_maroon, 1);
The above code will add a new room to the game and then set the background colour for that room to maroon and make the colour visible too.