IMHO, when dry brushing dark colours, such as RLM66 I would use a dark grey, something just a touch lighter than the 66, or add a touch or white to the 66. White or light grey will make to great a contrast, which gives the "sugar coating" effect. However, in well worn areas, such as the area in front of the rudder peddles and the seat I dry brush, fairly heavily, with aluminium, to show where the paint has been worn away. A wash can also help, of course you go the other way. I tend to use black for a wash on RLM 66. Rick's idea of using dark brown or blue would also work. Have fun :-)
Mal