I found my decal sheet binders in the downstairs closet this morning and, in one of them was the sheet I was talking about in my earlier post; Microscale sheet #48-24. You are correct about the YIPPEE being on the bottom of the wing and the decal instruction sheet so indicates. In addition to YIPPEE, it also has decals for GEN Straetmayer's personal transport (a droop snoot "L"), an OD/NG "J" of the 429th FS, and COLs McDonald's (Putt Putt Maru) and George Lavin's (Itsy Bitsy II) "L"s.
Upon reviewing the sheet instructions it does, indeed, say YIPPEE goes on the bottom of the wings. It also indicates YIPPEE should be painted Vermillion Orange, FS12197. I'm not sure how accurate Microscale's early color references were (or early color film, for that matter) but you can take it for whatever it may be worth. HTH.