I’ve just posted a new screencast in the Videos section. It’s episode 4, which shows how to set—and reset—your outgoing text color.
The process isn’t pretty; it’s highly non-obvious. The main source of problems is that whereas most other rich-text fields on the Mac go all rich-text or all plain-text, we juggle both. This won’t really work for the user until we put in a dedicated UI for this; that will come in a future version, but we haven’t decided on the best way yet. We do have a ticket for it; that ticket is #6349, which currently has a milestone of 1.1.
In the meantime, the current process is also described in text form on the FontsAndColors page on the Trac.