Thanks. The grey screen is not too bothersome as it only takes a minute to clear.
My problem is this also started with a 2009 Mac and eventually stuck at grey. That I don't want as it cost an arm and a leg to repair.
I ask why did this only start a few days after the upgrade ?
Safe sleep seems a stupid idea anyway and many clued up people agree with me. To do with saving electricity in Europe...
Why can't I just switch it off ?
Well, I think that I might have the answer.
El Cap wants to encrypt your computer by default, however the process to encrypt your SSD takes a lot (hours) and it is done on the background.
If instead of an SSD, you made the mistake of using traditional hard drives, you can expect the same procedure to take somewhere between 2x to 10x more. Remember, it also just happens when you are connected to a power source, like automatic backups in Time Machine.
It makes sense, doesn't it? Every time you fresh-install El Cap and the encryption process begins, you can watch it on system prefs - Filevault.
When I encrypted my slow external hard drive for time machine, one night wasn't enough for 100% encryption.
Like I said, the grey screen only happens because of encryption.