It sounds to me like your bobbin isn't threaded through the tension properly. Get your book out and be sure it is right, rewind a new bobbin if you have to. When you pull on the bobbin thread you should feel a slight resistance to know it is in the tensioner right.
Edited to add - be sure that there isn't any stray thread in the bobbin area. Take your throat plate off to be sure. Sometimes these can hide where you can't easily see them, but will interfere with the machines ability to make a stitch.
It is strange that it will do OK until you change thread colors. Are you using the same kind of thread for both colors?
Last edited by Craftnut; 04-01-2014 at 06:22 PM.