Because of the treated water. Even the hatchery that sell the minows use natural water until it reaches bait shop. Get home and put in raw well water or get some lake water and get temperature correct and try that. But even the dark gold wild shiners that shops sell will become almost a pale silver after a week or so in the bait tanks. They (wild shiners)also become very docile, best is shiners that are about 3 days out the river or lake, over the stress of being caught, still fiesty, but retain their natural color still.