You may want to try using greys instead of black. For a light to medium grey aircraft, a dark grey wash works well. For a white aircraft, a light to medium grey wash would do well. For a dark grey aircraft (or black), a light to medium grey wash would do.
Or, you may want to darken the base color of your aircraft with some black, until you get a shade that looks like the color in a shadow. You can also lighten the base color with white for "highlilghted" areas. Pixilater has some great weathering techniques.
On Bench: AM P-51B, Tamiya 1/48 F4U-1A
On Deck: Hasegawa F-14's (too many); Tamiya P-47D; Academy P-47N;