Once, a wise old guard designer told me that the uniforms reflect the concept of the show while the flags represent the color and the mood during each part of the music.
A few years ago, I designed uniforms for my guard that were shades of orange, yellow, and red. They just POPPED against the band's blue and white uniforms. I got compliments on it all season long. You want the guard to stand out, if the show calls for it. Its all about being a character, not being uniform as an ensemble anymore.
Just think about all the moments that would of been lost if the guard wore something that matched the corps/band... SCV's guard in 04 wouldn't have stood out, PR's guard in 05 in the purple and orange "showgirl" outfits, 00 Cadets in the individual outfits to celebrate their individuality. Now, think about what the show would look like if everything was matchy-matchy. I don't want to.
You know, if some costume/uniform made you curious enough to ask, then that designer did their job. I hope this answered your question!