Just do a little test. Mix it and test it out.
Seems that adding white won't go well.
Do you have Liquitex Matt Medium lay around?
You could try mixing you colors and then keep working it until the mixture just begins to dry out, and see if in that stage it will have the dry brushing effect that you are looking for?
I would try adding white like you said, but expect it to not work.
Then I would add Matt ModPudge and see if that would do the trick.
Then I would go and get some Liquitex Matt Medium to see if it will work.
Then I would give up and dry.brush it metallic and then wash it with indigo ink
Sorry, never done that before, just throwing out some ideas.