In just about every drill manual you will find instructions on the color guard and it's position in the battalion line. The color guard is to be posted on the left of the right-center company of a battalion (battalions with less than 5 companies are not supposed to carry colors technically). The right-center company of a battalion (color company) is supposed to be commanded by the captain that has the third most seniority of company commanders (the 3rd captain). Because of casualties, etc... that means that the company designated as the color company could rotate depending on the line officers present at the battalion level.

As for the color guard itself, the standard formation as listed in the manuals is supposed to be 9 men. This is either 1 Sgt. (known as the Color Sgt.), and 8 cpls, this is used for a battalion with 1 color being carried. In a battalion with 2 colors, there should be 1 Sgt (still called Color Sgt, but he does not carry the flag),2 Color Cpls (that do carry the flags), and 6 cpls. These men are picked by the battalions commanding officer from the best NCO's and usually know for their regularity in marching and drill (bravery also helps). This is how it should work as per the manuals...in the field however there are quite a number of cases where this system was modified, or went out the window completely. Hope this helps.