What do the different colours / icons on the note button mean?

The little note icon tells you the note's status. Normal document icon = saved and ready to open instantly. Amber/highlighted document icon = the task links a note that hasn't been backed up yet; click to fetch and save a copy. Archive-box icon = the original was deleted on your Supernote, but you still have a saved copy, so it opens that. Broken-link icon (not clickable) = the note was deleted on the device and there's no saved copy, so there's nothing to open.