Are you familiar with Acrylic Mediums? They are thick solutions you add to acrylic paint to change characteristics like opacity/transparency and matte/gloss. If you wanted to mute out some citadel colors you could try mixing something like Liquitex Ultra Matte Medium (adds matte characteristic, adds opacity) in to the paint, maybe along with a little black or sepia ink or paint. Also, if you haven't got your hands on some Liquitex flow-aid and distilled water, do it. Mixing in mediums and flow-aid and using distilled water immediately moved my painting to a whole other level.
If you're going to get these products, Les recommends DickBlick art supplies online - they are definitely cheaper than your local art store. Also, if you haven't read the post on how to make Les' washes - it's a must read.
http://www.dakkadakka.com/dakkaforum/posts/list/261541.page