You are correct about that. The greens won’t use a lot of power, and all WD internal drives are designed to run around the clock.
That being said, if you wanted a drive that “spun-down” you might consider a WD external such as a My Book Essential. They are designed to spin-down when idle. I’m not a big fan of these drive types myself, but if this is a priority feature for you, than it would be a better choice than to go with a third-party company.
Keep in mind that “build-your-own” solutions often have issues like this and its difficult to get support because the enclosure manufactuer will point at the hard drive, and the hard drive manufactuer will point at the enclosure. Thats the real advantage of buying a pre-built drive - both the enclosure and the drive are made by the same people.