Hinksy: To give proper credit, that Panzer #215 posted a few responses back is DoogsATX...a fine build. wbill76 nailed it with the contrast thing. I am a real fan of washes (as I wrote to you earlier) and I found (on my current Tiger Mid) that when I used my typical colors arsenal the white camo and the dunkelgelb grabbed the colors equally, as you would expect, and I was VERY disappointed that I lost the contrast that I liked so much in the original paint job and chipping effects using the AK fluid. I had to go back (more than once) and add white oil washes, of different dilutions, OVER the areas of white camo until I was happy with the look. This was a step I expected to do anyway but it would have been later on when I was applying the local washes.
I guess my main advice (if you go with the dunkelgelb) would be is that you need to really discern the level of white, or the amount of wear, you want to depict. For a more white look I'd keep your initial washes (I like the word filter) to be in the cooler color ranges and keep it limited and very thin, on the dunkelgelb...heavier on the white. For a more worn overall look , I'd use a VERY thin initial overall wash of raw umber and then add a second of the same color over the dunkelgelb only. Thin washes REALLY help achieve the effect you want with a lot of control...and don't be afraid to try a local thin pure white wash! It'll bleed over into some areas you may not like and you'll probably think you've ruined your model when it's wet but hang in there and wait until it dries completely to really see what you think. It's my opinion that the real stuff would have been diluted by the elements and would run like that anyway.
I personally don't like to go back and "fix" previous builds but if want to try the whitewash on the SiG first I think that's a great idea.
Good Luck, and be sure to post pic's!!!