ID:883572
 
(See the best response by Albro1.)
Say you used night-vision goggles and you wanted the map to be tinted a shade of green, representing night vision. How would you do this?
Create a partly-transparent green icon and put it on a client's screen. Set it's screen_loc to SOUTHWEST to NORTHEAST.

NOTE: With this method, you should probably use a partly-transparent black icon to represent night-time. Replace that with the green icon and then viola, you can see better than others.
One method would be to have a translucent green icon and create a single universal object at the start of the world to be used as a HUD.

This object (an /atom/movable, that being either a /mob or an /obj) should have screen_loc defined as "SOUTHEAST to NORTHWEST" to cover the whole client screen.

You should set its mouse_opacity to 0 so that this object is ignored by the mouse.

When the night vision goggle is activated, you simply add the object to the client's screen as you would do to add an object to a list (like contents). Once done, remove the object from the client.screen.
I've had trouble before with screen_loc. I could never figure out how to set things to screen. How do I do that?
Best response
obj/hud
screen_loc = "SOUTHEAST to NORTHWEST"
icon = 'night-vision.dmi'
mouse_opacity = 0

var/obj/hud/night_vision

world/New()
. = ..()
if(.)
night_vision = new

mob/verb/Night_Vision_Activate()
usr.client.screen += night_vision
mob/verb/Night_Vision_Deactivate()
usr.client.screen -= night_vision
Thanks this helped me figure it out, appreciate it.