Hey PolarBear,
On my P-51 with metal finish, I went back and applied a wash of water color paints. I thinned it way down and applied it very thin. I used water colors because
1. I new that the water colors wouldn't effect the paint, as this was the first metal finish I'd ever done.
2. With water colors there is no mess if you make a mistake (like I usually do).
You say you are looking for a very subtle effect. That is what I was trying to achieve also. Having built the model, I can tell the panel lines have been accentuated (spelling) but anyone else would probably never notice this since it is very subtle.
This probably isn't the ideal method but it worked pretty well for me.
Hope this helped a little
Hermon