That looks good! Do you plan on going back over it with other colours? Because a couple of layers of that and you'd have a really effective camo going on.
I've also used salt weathering on my Imperial Knight. It's a really good method of getting a nice well-worn effect, that's relatively low on effort.
More pictures in my gallery if you want more angles/close ups.
From my experiences with it.
-Rock salt is the way to go. Buy a bag, grind it up in a pestle & mortar. Or stick it in a zip-lock bag and bash it with a hammer. This lets you get a wide range of variance in size and shape of the salt. Which in turn gives you a more natural appearance to the weathering.
-I found hairspray works better then water, especially with the larger chunks of rock salt, which tend to fall off once the water dries a bit.
-I used rattle cans instead of an airbrush and I also had it blow salt off. Luckily I was going for a heavily weathered effect, so I'd already over-salted. But it is something to be aware of depending on how beat up you're aiming for.
-I used a grey primer so that once weathered, it would look like the paint had stripped back to bare ceramite. And while I am happy with the final look of it, it does kinda look like I just weathered straight on bare plastic. So that is also something to be aware of as well.
If you want to primer grey, then you could remedy this by giving the model a light dusting of black to help add in some tonal variation. This would work either with a rattle can or an air brush. Although you would certainly have more control with an airbrush. This is what i'd do if I ever painted up another Knight in this colour scheme.
It's a great technique and definitely one I recommend people try.