Oops! No problem. You'll need to close JDarkRoom, find your jdarkroom settings folder, which on windows ends up somewhere like C:\Users\Lisandro\.jdarkroom
(note the .
If you don't mind losing all your settings, then simply delete the jdarkroom.properties file and then start JDarkRoom again.
If you want to keep your other settings, edit jdarkroom.properties in another text editor (e.g. notepad), and delete the line that starts with 'colours.foreground'. Save your changes, quit notepad, then start JDarkRoom.
Hope that helps,