With the resurrection of labels in MacOS X 10.3, I hastened to add a little program to the osxutils package. It's called setlabel and allows you to set file labels from the command line with a very simple syntax:
% setlabel Red myfile.txtI also updated lsmac and it now is capable of listing file labels, along with File/Creator types, Finder flags and such.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031030221241557