Western Digital's Green hard drives, known for their combination of high capacities with reasonable performance and low noise, are no more—at least not in name. The company says it's folding the Green drives into its Blue line-up to make it "easier for customers to choose the right drive for their PC."
In practice, the new Blue line-up seems more confusing. Before this rebranding, Blues represented value-oriented 7,200-RPM disks with slightly slower performance than the top-of-the-line Black series (at least on the desktop). Now, buyers will need to distinguish between "5,400-RPM-class" and "7,200-RPM-class" drives under the Blue umbrella for some capacities. WD's product page suggests that the easiest way to do so is to look for a "Z" at the end of the model number. For example, the 3.5", 1TB, 5,400-RPM-ish Blue drive is the WD10EZRZ, while the 1TB, 7,200-RPM drive is the WD10EZEX.
Be careful that you're getting the drive you really want next time you go parts-shopping, and check out WD's Blue product page for a full accounting of the affected models.