How do I recall the last sent message in Colloquy?
Per Colloquy's FAQ:


  Is there a way to see my sent history as well as to navigate around in the existing text with the arrow keys?
  Sent history is controlled by the up/down arrow keys while holding option, switching between panes in a window with command-up/down, and scrolling with home/end/page up/page down; you can use any other key combination to navigate or select text. Change this within the hidden preferences.


So; &#x2325;+&#x2191; or 
&#x2325;+&#x2193; gives you the last, or next message, respectively.

And per that Hidden Preferences link, it would appear that you can change it to use up/down arrows only by opening Terminal.app (either search for Terminal via Spotlight, otherwise you can find the app in Applications &gt; Utilities), and enter:

defaults write info.colloquy JVSendHistoryUsesOnlyArrows -bool true

Restart Colloquy after entering this command if it was already open, and see if that works.