Tom Kelly and John Dilley were the first to paint a Mustang in this manner, with that very aircraft (44-73436), as early as 1977, and a number of other owners followed after finding the blue/yellow combination attractive. At that time, it was painted as "Lou IV" E2-C and registered as N51KT. I seem to recall that the decision to use the powder blue paint for the upper surfaces was a bit of an effort to thumb their noses at the warbird paint critics, but I'm not sure. Another early one, painted in much the same way, as early as 1978, was Don Weber's P-51D 44-73254 (today "Louisiana Kid"), which was painted as "Buster" with the same powder blue upper surfaces/361st FG. When Kelly and Dilley sold 44-73436 around 1985, they purchased P-51D 44-63350 and had it painted the exact same way, as "Lou IV" E2-C with the blue upper surfaces and also registered as N51KT (by this point, their previous Mustang was now registered N51KD, as it remains still today with the Olympic Flight Museum). Other Mustangs restored in psuedo-361st FG markings with powder blue upper surfaces followed through the 80's and early 90's, with examples like Bob Pond's P-51D, Paul Poberezny's TF-51D "Paul I", the P-51D "Six-Shooter", the P-51D "Obsession", the P-51D "Dazzling Donna" (today "Stang"), and probably a couple more I'm forgetting. No one at the time really believed this is how the aircraft looked during WWII, they just liked the way it looked. There was a debate at one time, at least up until the 80's/early 90's, between whether the 361st used the much darker insignia blue or RAF dark green/OD for the upper surfaces, but that debate has long been put to rest.
The paint scheme on the Olympic Flight Museum's P-51D "American Beauty" has arrived where it is at through various modifications of the aircraft's paint scheme from way back in 1977. At that time, it was painted as "Lou IV", with much of the nose painted yellow, the code letters E2-C, and the serial 413410 on the tail (code/serial details correct to the wartime "Lou IV"), but the rest of the base scheme has remained largely the same ever since. When the aircraft was sold to Dean Cutshall, the nose was repainted with only the area forward of the exhaust painted yellow, the name was changed to "Cutter's Capers", the code letters were changed to E2-S, and the tail serial was changed to 413926 (code/serial details correct to Urban Drew's P-51D). When Wally Fisk owned the aircraft in the mid-90's, the name was changed to "Slo-Mo-Shun", but everything else remained as it was. When Bryan Reynolds/Olympic Flight Museum purchased the aircraft, the name/nose art was changed to "American Beauty" (which comes from a 31st FG Mustang), and the personal details of Col. Ted Bullock (52nd FG pilot, and a resident of Olympia, WA, where the aircraft is based), who has now since passed away, were added below the canopy. Otherwise, the rest of the markings have largely remained the same (though repainted/touched-up over the years)