I made this change when topcolor was first introduced and have never messed with it since.
also, you can check https://news.ycombinator.com/topcolors as the list of most popular topcolors.
I have a custom CSS file using Stylish and my HN is mostly greyscale. (Topcolor is #AAAAAA.)
However setting a topcolor is a waste of time as the "Add Comment" page doesn't obey it, and the Y logo looks pretty horrible with anything but the default.
It's significantly different from the stock orange, but it complements the logo well. I only care about it because it lets me know whether I'm logged in or not.
Cuz it's wavender. Duh.