A friend of mine is a recent newbie Mac user, after finally succumbing to the shiny white lure of the MacBook. As an expert Unix user, he was missing the old X11 colours list, because he can't have his terminal set up with a horrible shade of puce without having to work out the RGB values for it.
Christopher Holland has already created an OS X color list for the Resene color list which is allegedly used on many X11 windowing systems, but to be honest, I haven't encountered it before. I last used an SGI in about 1997, and I must confess I never looked at the color selector. It also didn't seem to include the specific wretched tincture my friend wanted.
So, rather than taking the easy route and just picking the closest match from the Apple crayons, we had to grep /usr/X11R6/lib/X11/rgb.txt instead.
To eliminate the possibility of this nightmare operation having to be done again when he finally comes to his senses and picks a normal color, I hacked together a quick bit of code to create a .clr file.
Just use the "Open..." doodah in any colour selector in Mac OS X. It should automatically copy the file into your user's Library/Colors.
UPDATE, March 2007: I got an email asking me for the code to do this. I must admit I threw away the ObjC code I wrote after running it (stupidly), but the request catalysed me into looking at another approach: using PerlObjCBridge. The code is now available here in my Subversion repository. In the process, I also made the code write out two clr files: one with all colour entries (including duplicates) and one without duplicates.