Why are cavalry guidons/flags red and white?

Actually the red over white swallow tail pennants predate the reasons you state - they were originally carried by lancers in the 16th Century - later became the lancer pennants in many Napoleonic armies - almost always (however there were some exceptions) RED over WHITE - even the Polish lancers whose national colors were white over red - bore the RED over WHITE pennants. Somewhere along the line of history these pennants became guidons adapted by the MOUNTED TROOPS (cavalry and light artillery) . During the American Civil War the guidons were replaced - (1862 regs) due to the fact that they resembled the Confederate fist national colors (stars and bars, red-white-red) and replaced by Stars and stripes in swallow tail design. Sometime after the war red over withe came into use again - not to say that many units never discarded the red and white guidons in the first place but like many Civil War flags they were often used until in total tatters . . .