I said it backwards...possibly bobbin thread too tight, top too loose?

I wonder if it is the bobbin thread. I think you should try pre wound bobbins like Linda r suggested before I messed with tensions. Neither of my machines like the embroidery thread in the bobbin.

I have a Brother LB6770 and mine has done the same thing. I found that it is not seated in the tension disk. It could be so many things. Just keep trying and you will figure it out.

In looking at your photos, I am guessing it's a threading issue, because it works fine until you put in the pink and/or black. Is it possible there is something different about the weight of the different colored thread? Or is it possible the tension somehow got messed up at the thread change? Or is it possible that you didn't get it threaded properly? Did you remember to put the presser foot down (ask me how I know about this one!)?

I tend to get in a hurry and not make sure the thread is where it belongs. Also, if you are switching out the bobbins, they may not be right where they need to be.

Also, check out the thread. Hold the spool up, let the thread fall down about 12". Is it falling in a nice smooth line or is it twisting and turning and kind of gritchy looking? Whether it is polyester, rayon, or metallic, it should be falling in a smooth line. If it isn't, then you need to change to a thread that is. How it looks is how it will be feeding through your machine, bobbin or top, and it will cause problems if it's not a smooth line.

Also, I have owned 3 Brother embroidery machines, loved them all, but I have found the bobbins are not thread friendly with some brands. I've had the most consistent luck with the OESD bobbin thread. It's a little pricey but I buy the 6,000 yard spools. I also use Sulky bobbin thread. They both work well.