Don't know about the yellow in the bars, but the red outline was added, with the bars, to make them more visible. The bars were added to present the insignia as a rectangle at a distance, as the star and disc could easily blur into what could appear as a cross or merely a circle. However, after complaints from the PTO, where it was thought the red could be confused with the Japanese markings, the red outline was ordered to be overpainted in blue. The order to add the bars was issued on June 28th 1943, to be implemented by September 1st 1943. The order to change from red outline to blue was dated August 14th 1943, only two to three weeks after the initial changes began to take place!
The specifications for the bars were that they should be a one circle radius (of the blue disc) either side, and one half the radii deep.
The later toning-down with grey was a measure to overpaint the white areas, mainly the bars, in a thin wash of neutral grey, in order to render them less conspicuous, paricularly on bombers painted in Olive Drab, and it seems that this only lasted a relatively short priod of time, but, again, with examples cointinuing in service due to operational pressure.
I've never seen any specifications, or details of dates, theatres etc, for the yellow area within the bars, and wonder if this has perhaps been mis-interpretation of photographs at some point?