Look for plump, smooth grapes with good color. They should be firmly attached to a fresh-looking green stem, with no evidence of wrinkling and withering. There should be a dusty bloom on the skin of the grape itself. Like the dusty bloom on blueberries, it's a naturally occurring substance that helps protect the grapes and is a good indication of freshness. Green or white grapes will have a golden glow when they're ripe, red grapes will be a soft, rich red, and black grapes will have a full, deep, blue-black color.
