Conker's shoulders lightened as he got older. He also used to have a distinct solid dark red line down his back but it began to lighten and now you don't notice it as much unless you are looking for it.
Shibas tend to change colors a lot as the mature. If yours wears the harness a lot though it might be rubbing some of the top coat off so it appears lighter.

Is it possible that the harness is wearing/tearing the top coat, revealing the lighter layer below? I don't know if this is what you are describing, but it doesn't sound like the fading shibas get from "greying" as they age or what mine is getting from his near-constant sunbathing. My shiba's collar tears up his fur, so I don't have it on him when we are in the house, I just put it on when he goes out.

LOL shibahiro! Jack is a B&T and the harness was rubbing off the top of his fur - they are tan lines! we switched to a martingale collar; his tan lines aren't darkening yet, but maybe after he finishes shedding all his fur and by winter his tan lines will fade (or darken, I guess.)