Thanks for the excellent tips! I've already bought some dust for weathering the Maquis raider, but this sounds like a far better method. If I buy a pack of colours, it'll give more options for mixing and getting the right tone. At least it seems that way to me.
I also picked up a Tamiya weathering sponge brush. It's like a q-tip (we call them cotton buds across the pond), but obviously needs to be kept clean like a brush.