All plums share three common characteristics: thin skin, a single central pit and a succulent flesh when ripe. Colors vary depending on variety. Fruits can be deep purple, rose-hued, ruby red, green and gold. All plums continue to ripen once harvested. Plums truly abound with over 2,000 varieties of plums in existence, and over 100 are available in the United States alone. If you want to purchase plums that are ripe and ready to eat, look for ones that yield to gentle pressure and that are slightly soft at their tip. Select red plums with smooth skin that is free of bruises and cuts. Look for plums that still have a dusting of white. This substance, known as the "bloom," is a testament to the plum's freshness. Regardless of color, a ripe plum should yield to gentle pressure, especially at the end opposite the stem. Good-quality ripe plums should have a sweet fragrance that is truly reminiscent of a plum.