Color is a great indicator of maturity. Peaches are ripe when the ground color of the fruit changes from green to completely yellow. Some of the newer peach varieties have a red tinge to the skin, but this is not a reliable barometer of ripeness. There is a fine line when harvesting peaches. You want the fruit to hang on the tree long enough for the flavor and sugar content to peak, but not so long that it becomes overripe.