Plums come in many varieties and most are grown in California; the Casslelman, Red Beaut, and Santa Rosa are some of the good varieties. A plum's ripeness can not be judged by its color because each variety has its own color: Santa Rosa plums are dark purple in color, Red plums are redish-purple, and Black plums are deep purple almost black. Yet all plums should be soft when ripe and never hard. The softer the plum the sweeter the plum.