Instead of peat, you could maybe try some coconut/coir(?) fiber. sometimes you can find this for orchid mix, or in chunks for hydroponics. I'm not sure it would be lighter (I think so), but at the least it should be a different shade, which should differentiate it from the peat-mix that makes up the tree. It may be a bit more difficult to use.
I'd recommend some sort of sand, but that would look more like rocks and less like vegetation.
It looks like the rope you are using is braided- you may be able to stick the wire down the center (though, this could be difficult on the longer lengths),
btw, it looks really great, congrats