The color signifies what day of the week the bread was baked on. The practice of using these color codes is not meant necessarily to be used by the customer, but actually is to aid the person stocking the shelves with break in determining what bread is old and needs to be removed. This way, they don’t actually have to look closely at the tabs, which usually also show a “sell by” date. They can simply just look for one specific color and remove those. Each manufacturer maintains their own color code scheme, but this is usually easily discoverable on the web, by asking a bread stocker at the store, or by simply emailing the manufacturer of your favorite bread.