For USAAF CPs, I like Interior Green (Tamiya XF-04) - BUT, it’s not always correct. Some factories used one color, some used another.
Your best bet is too pick up some reference material and just look at the pictures. For instance, my HellCat uses interior green, but my Jug uses regular Tamiya green. From the pictures I studied, regular green was the absolute best match I could find - and it looks dead on.
One way is; Future, decal, future (then dull coat if you want a flat finish)
Another is; Future, decal solvent (micro sol), decal, decal setting solution (micro set), future (then dull if you want a flat finish).
The micro sol makes the decal much more flexible and really helps them to conform to diferent shapes. The micro set pulls the decal down into the different shapes and makes the decal look painted on.