Board Thread:Speculation House/@comment-4830106-20130906015252/@comment-16774229-20130906024828

Angren Túrë wrote: There really needs to be a Dust-centric episode so we can all sit back and say "So thats how everything works!" This what I think the color change means.When Weiss made the Dust glyph, it was wight becasue it was created with and sustained by her Aura, and wight's her color. it turned black when she cut it off from her Aura, and red when she linked it to Ruby's.

So basically, powerful Dust techniques use Aura, which is why it could defeat the Grimm. They can pick up a sword, or a gun in some cases, and use it against you, but Dust, they could never use to it's full potential, because they lack Aura.

At least, that's what I think it means.