Ive noticed that in late WW2 USA fighters, some had a red outline around the stars and bars,blue and white insigina, Why?

propboy44256 - check out this link may give you some insight.
http://www.njahof.org/insignia.htm

The red outline was on US aircraft between 1942 and 1943

The prewar US aircraft insignia, as I'm sure you know, was a white star, centered on a blue disk, with a red disk centered in the star. Early 1942, the red disk was removed to avoid confusion with the Japanese hinomaru. August of 1943, two white rectangles were added to either side of the blue disked star, and the whole thing was outlined in red. Again, because of the fear of confusion with the hinomaru, the red was replaced with blue. That happened around October of 1943. Sorry, I don't have the exact dates in front of me. Of course, because of busy operational schedules, there were red outlined insignias on some aircraft through the end of 1943. Information from Roger Freeman's "The Mighty Eighth." Also, the insignias were removed from the upper starbord, lower port wings to confuse the aim of an enemy aircraft. It was found that the two national insignias would naturally provide an aiming point in the middle. However, P-47s with the 8th & 9th Air Forces carried the insignias on both bottom wings to avoid confusion with the FW-190. Also, naval aircraft carried the insignia in all four location through early 1943. There may well have been other exceptions.

Best regards:
Charlie