I recently posted this question on SuperUser: https://webapps.stackexchange.com/questions/90176/conditionally-format-data-in-spreadsheet-column-but-only-when-it-changes
I did this because the question seemed like a fit for the site and when I went to tag it there was even a handy google-spreadsheets tag auto-suggested for me so it seemed like a no brainer.
But then I was made aware that this is off topic.
How do I know something is off topic when I see a tag for it completely describing what I am asking about?