I think OP is refering to the graphical representation of the icons on the hud and not the actual damage, this is something i have wondered myself to, they do get completely red when totaly destroyed though, wich made me think that they might only hav two states, but if you @Stazco say that there is wheel health to be looked at, that is maybe not the case.
I agree with @LordEisi . Damage indicator is somehow strange. Yes, we have several wheels on the truck. But it only goes red, when one of the wheels destroyed completely. Otherwise, if there are even 1 hp on each wheel it will show everything OK. You can only see wheel health through function menu.
Also you cannot change a damaged wheel. Only completely destroyed one. Let's say, I have a truck on remote location (far from garage) that I'm going to load and go for a long trip. I have a forward wheel badly damaged 10/50 hp. I have an ability to use service trailer before I start my long trip. But in the current game mechanics I will not be able to change the wheel preemptively. Though IRL I would do so.
Instead I have to crush to kill the wheel completely, then replace it with new one (and repair other damages) and only then start the trip.
We cannot repair from not attached service trailer, though we can refuel from not attached fuel tanks. As well, we cannot fill parts from service trailer to internal truck repair kits or other trailers.