According to"Reader's Digest", commercial bread is baked and delivered on a strict schedule to keep it as fresh as possible, and the color of the tag on the bag indicates the day of the week it was baked.
Even though there's a "sell by" date, the colors are supposed to make it easier for the staff in the store to rotate fresh bread IN and older loaves OUT. And because of that, you'll probably only see two colors on any given day.
Bakeries usually have Wednesdays and Sundays off, so there's only five colors: Blue is Monday . . . Green is Tuesday . . . Red is Wednesday . . . White is Friday . . . and Yellow is Saturday.
There's an easy way to remember that . . . the names of the colors are in alphabetical order by day of the week. (So, B, then G, then R, W, and Y.)
Of course, not EVERY supermarket and bakery uses this system, but it's widely-used at the large chain stores, and some smaller stores too.