Actually Casey's Tactics is the book that you need to refer to instead of Hardee's as Hardee's only calls for one flag and if you are reenacting Union Army then they carried two flags and the color guard is a bit different.
Casey's was a copy of Hardee's (no self-respecting Yankees would use a drill manual from a now Confederate general) and when you form the regiment in line it will be the same basically - the color guard and guides are posted where the colonel wishes to form his regiment and then the regiment forms on them. With the color company being in the center of the regiment this would, of necessity, place the flags in the center (roughly) of any column formation so that when they shook out into line of battle the colors could be quickly placed in the center of that line as the regiemnt formed on them.
I believe the color guard formed the left hand corner of the color company. I do not recall any specific hand holds ever stated for carrying the colors but the color bearers typically had color belts to help hold the flags since Union flags were 6 by 6 1/2 feet overall - much larger than Confederate colors.
Hope this helps some.