This is common bread mold. Any other colors seen are from other varieties of mold. If you look at it under a magnifying glass or low power microscope, you will see that the white material is actually threadlike. Some of the mycelium branches upward, ending in sporangia, or spore cases, containing many spores that will eventually burst free and float away to form new plants. Discard the bread after examining.
Yeast is also a fungus, and gives off carbon dioxide as it grows.
If you put a little baking yeast into a cup of sugar water and let stand in a warm place for a few minutes, you should be able to see the yeast plants budding to from new plants. Water normally enters a plant through its roots. It is carried upwards to all parts of the plant by means by capillary action. You can make the tubes that carry water through a plant easier to see with a class of water that has been dyed with food coloring. Let the carrot sit overnight.