After plums have been picked and stored for some time, they will increase in color. They do not increase in maturity regardless of the color, and therefore whenever a lot of plums arrives at the packing house and is immature, it must be reconditioned immediately. It would be impossible within a relatively short period of storage to determine which of the individual plums were immature unless sorting is done upon arrival at the packing house.
Cal. Code Regs. Tit. 3, § 1462.4