It was a practice started in the 8th Af on B-24s... The fuselage insignias had the star & bar "greyed out" to deny the Luftwaffe pilots an "aiming point" on flank-attacks. It subsequently went on to be applied to all types, but sheer numbers prevented it happening to all aircraft in the 8th, so instead of a general order, it was left to individual squadron & group COs as an option.
This followed the removal of the roundel-star insignia from all US aircraft from the right wing upper and left wing lower surfaces.
This practice was also to deny enemy pilots an aiming point of "Shoot between the stars" to hit the cockpits during a high or low-side deflection attack...
The Star & Bar insignia was put back on the lower right wing of some P-47s after D-day to increase recognition for Allied anti-aircraft gunners due to the Jugs being utilized heavily in ground attacks ("Rhubarbs") in proximity to Allied ground units.
The red center on early US insignia was removed just prior to the Battle of Midway, to preveant friendly-fire incidents by gunners mistaking the red center for Japanese hinomaru. Ted Lawson mentioned this in his book, "Thirty Seconds Over Tokyo", as an explination of why Japanese civilians were waving at his B-25 as they flew overhead. (More likely that they just assumed any aircraft over Japan were theirs, though)
The addition of the red surround in 43 was short-lived because of PTO airgunners' reports of them still "seeing red" and firing on other US planes in the heat of battle (That's also where the expression of "makes me see red" came from)