Devo - with high contrast camo schemes what I do is after all painting has been done I use a VERY thin (like 5:1 thinner to paint ratio if not more) coat of the lighter paint color with a dab of white to lighten it a tad more than the original, and from at least 6" mist it over the entire upper fuselage and on the sides, VERY slightly overlapping the demarcation line at the upper/lower paint colors...
If the underside is another very contrasting color (i.e. RLM Light Blue lowers with RLM Sand yellow and RLM dark green on top) I take the same light mix of the lower color plus a tad of white and same kind of mist over the lower surfaces slightly overlapping the demarcation line again...
The paint should look like colored thinner, with the sand yellow I spoke of it looked like a slightly chalky water in the spray cup...
here's an example on a Martlet I did based in N Africa..
before spraying it looked like a Reno racing aircraft...
Here it is weathered after the light top coat
This blends the colors a bit making it look less "new" and toylike. When you are done you coat it with future and weather away!!! This also helps reduce the contrast... Andy and Simon already covered that for you...
P.S. the tannish streaks on the sides are pastels.. I was trying to immitate the accumulation of sand blown across protuberances on the fuselage.. it doesn't have that much contrast to the eye.
Good luck!!