Hi, the only thing I've ever done is add pureed spinach to my dough. It wasn't perfect, but it worked for me. I started with the basic mix in the book that came with my attachment, but I can't find it, so I now use the recipe in Marcella Hazan's book, Essentials of Classic Italian Cooking. I was perfectly happy with the recipe in the book, though. So if you get a chance, I'd love for you to post it. It was a softer, more manageable dough. I have a feeling it has a lot to do with the kind of flour she uses.
Also, Gina, our newest member gave me some recipes for flavored dough. I haven't gotten a chance to try them since I got my pasta attachment. I'll let her post her recipes, though. She may even have some new ideas!
Another tip I've heard, but not tried, is adding baby food to your mixer when you're making up your dough.
I haven't played with my pasta in a while. I think I'll make that my August project. William has complained that it's been such a long time since I've done it. It is SO much better than store bought. Another tip, if making lasagna, I don't even cut my sheets. I lay the raw dough out over my layers, slightly overlapping. GREAT STUFF! Have fun!