Time Machine Disks No Longer Green?

Under my previous OS, the Time Machine Disk icon on my desktop was always green with the TM icon on it. However, since I installed Mac OS 10.11.6, it is the same featureless orange icon as the rest of my external drives. I've checked and it is selected in the TM control panel in System Preferences, plus it is still backing up as scheduled. Everywhere else it is green: Get Info window, TM control panel, Disk Utility.

Any idea as to why the desktop icon is no longer green?