All duns are "line-back duns". I'm not sure where that name came from because a horse must have a dorsal stripe, in addition to other markings, to be a dun.
Steel dust is not a color. I'm not sure where you got the term (if someone else said it earlier in the thread or someone told you it was a color). It may have been used to refer to a shade of gray in the graying process, which isn't uncommon (ie, rose gray). But all gray horses are just that: gray. Rose gray or "steel dust" is nothing fancy. It's still a gray horse. ETA: A google search of "steel dust horse" brings up pictures of a horse named Steel Dust and a few grullas. No, those are grulla horses. Nothing special about them.
And the non-silver grulla mentioned previously is not referred to as silver because there is a color gene named Silver that acts different, although that grulla may actually be colored what we consider to be the color silver.