This is another way, originally commented by murray, but deserves more attention, so I'm posting this after testing:
Suppose, you want a command completion for \textcolor{red}{coloured text}, which can be used to output coloured text in colour red (to be used with \usepackage[]{xcolor} in the preamble.
The GUI way (with less control of course): type and select \textcolor{}{} in the command window, then press command+shift+W (shortcut for: TexShop menu > Source > Command Compeltion > Add Word). You're now ready to use it on the fly, without restarting your work/ TexShop.
The command completion editor way (If you want more advance functionalities like: the cursor jumps to the first {} while autocompletions is triggered) Go to TexShop menu > Source > Command Compeltion > Add Word. Add this: \textcolor{#INS#•#INS#}{•} at the bottom of the file, press command+s & you're good to go.
I believe, if someone takes a look at the entries present already in the editor, he/she can automate all the autocompletion needs by adding simple entries. texshop is very versatile & powerful editor, with nearly all features built-in, IMO.
Cheers!