Darrell Larson writes in, concerned about his Time Machine backup:
For the last several Mac operating systems, including El Capitan, when I restart my iMac, my Time Machine Backup remains yellow on the desktop and does not turn green. But then one day it turns back to green for no known reason. However, with El Capitan it continues to backup every hour as scheduled.
In other words, everything is working even when it looks like it isn’t.
Turns out, this is a very longstanding problem. I can find references to it as far back as 2007 and as recently as a few weeks ago across many forums, including Apple’s. From all accounts, it’s cosmetic: The default (non-custom) drive icon for Mac volume is orange; when you anoint a volume as a Time Machine destination, it gets a fancy green tone with a Time Machine symbol embossed on it.
Read more here:: What happens when your Time Machine drive icon changes