Many of the colors you see on the moldy bread are due to the spores the fungi have produced. Molds reproduce by making spores at the end of stalks that rises above the surface of the bread, giving molds a fuzzy appearance.
Spores are usually colorful.
Some of the colors on your bread may be the result of growing colonies of bacteria, which also sometimes grow on old food. You can tell bacteria colonies apart from molds because bacteria colonies appear smooth while molds look fuzzy.