Answer: I am inclined to think that the plums on the tree are not quite ripe and when they are fully ripen they will be all the same color. The parts that are lighter color are probably the sides that receive the most sunshine. Some plums naturally have an almost 2 tone coloration. I'd just wait and see how they mature. Taste is the best way to tell if they are ripe. About the only way to identify the type is to take a couple of ripe ones into your local cooperative extension office for identification. Or maybe there's you can find a local branch of the California Rare Fruit Growers club. (http://www.crfgsandiego.org/Newsletters/CRFG-SD-Newsletter-2...) It's a national group of backyard fruit tree growers and they love to share their knowledge. Best wishes with your plum tree!
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