Hi Lance,
All the oil paints I used are from the Artists' Oil Colour series by Winsor & Newton. For the specific Fokker I used a mixture of 2+2+1 of Olive Green 447, Davy's Grey 217 and Burnt Sienna 074. The result looks a bit dull on the mixing palette, but is ok when applied. However, after previous attempts with this effect, I have come to the conclusion that the exact tone is of secondary importance. The most important thing is the application, and especially the use of the second "erasing" brush. I apply paint with the first "painting" brush in a small area, wipe of with the "eraser", clean it and repeat next. From time to time I erase twice so as to have darker and lighter areas. The specific triplane old pictures show it was heavily streaked to the front and lighter to the back, while the latter production Dr's don't show this to the same degree. Most of the job on the model was done with the second brush. The good thing with oils is that they do not dry, so you can always wipe it of and start again until you're happy with what you see :-D
Regards !!!