In reply to
Oh, Dopplr colors. The Dopplr blog is archived in the Internet archive. Dopplr discusses their use of colors for cities here: web.archive.org/web/20080…
There is a GitHub repo (not by Dopplr) that tries to echo their approach: github.com/banterabi… . Ian Kennedy blogged about Dopplr colors at the time: github.com/banterabi…
There is a really good post at optional.is: optional.is/required/…