I had the same problem of only printing in yellow. I got to the solenoid and this was the first time I ever saw one, but it looked brand new to me and the felt looked fine. I scraped it off anyways and put a new felt that was a tiny bit thicker. Bingo it worked. I also found this youtube video very helpful in figuring out what the solenoid looked like:
http://www.youtube.com/watch?v=9BoxmagIjhg
And also here is another video which although is for a different printer, explains what is happening and why very well: http://www.youtube.com/watch?v=LtJbJw6m34I
Even though mine looked fine, it was sticking.