After many iterations, the camo is finally done. The results of my first try were a bit un-nerving, as the "hard-edge" camo looked really bad and the color looked even worse!
Then I went to walmart and bought about 8 packs of silly-putty to try the "soft-edge" method for the first time (see first picture, where I am over-painting the original coat). A few more applications were necesary until I finally arrived at what looked like the right colors for sand camo. I think the end result looks pretty nice, even though it's not perfect. I expect to heavily weather the aircraft, so hopefully any blemishes will be less noticeable afterwards. Note that I still have to paint some of the canopy frames.
That's a gorgeous P-40, Aggie!