I'd like to make some notes/mark some cells privately on a shared Google Spreadsheet in Google Drive. Is this possible?
I figured out a work-a-round and this is how to do it:
Shared file with edit rights
Now you're able to make notes without anybody seeing them. If you want to see them, select the cell to see the content of that cell. It won't appear in the cell itself, but to the right of the Fx symbol at the top of your screen:
Other users can't edit the sheet, thus can't control the font color.