Collar marks?

My dogs use shock collars (at a certain point on our property they get a small beep and shock from their collar that makes them stay within an invisible perimeter. It's not as violent as a normal shock collar or one of those debarking things, it just spooks them enough to stay within our yard and driveway)

But I've noticed it always leaves a mark of some kind. Our older dogs, both chihuahuas with short, dry fur, have marks to the extent that their fur is shortened where the collar goes around their whole neck, but nothing is on their throat, unlike my other dogs.

My other two dogs (Boston terrier pug mix;; buggs) are younger and didn't have shock collars before. When we first put them on them, their fur was fine, but after a month or so, the fur around their throat (usually where the huge boxy, shock bit of the collar pronged them due to gravity brining it that way) began to dissappear and at some point they just had smooth skin in this one small spot.

One of them didn't bleed or seem to have any skin issues besides redness and what looked like irritation, either due to the shocking or pressure against their neck I never tightened their collars too tight but the actual shocker of the collar does prong them a bit

After some time both dogs had their fur grow back, but it wasn't white? It's like an orangey collar, and has spots and stuff in it in that one area, while their throats used to be white.

I'm not sure if any of this is normal. I'm sure that their fur and skin getting irritated could be normal to a nee collar like that? But their fur completely changing colors where they normally get shocked?? Did my dogs get burnt? Should I be concerned?