The 15000 has a choice of three different bobbin holders, red dot, yellow dot and blue dot. The main difference is the tension setting. To obtain the correct stitch you need to use the correct bobbin case as well as the correct needle. You should use this chart as a guideline. You may not need to use the blue dot bobbin holder if when free motion quilting your thread does not pull up to the top or pull down to the back of your quilt.
Thinner thread such as 60 weight will need a bobbin holder tension that is higher. When using the thinner thread use the yellow dot bobbin holder. Janome prewound bobbin thread is 60 weight.
The 15000 came with a bobbin holder cleaner. Page 158 of your manual explains how to use the cleaner.
Click on the link below for a PDF copy of the chart. This chart is a guideline.
I thought the yellow dot was for use with non Janome bobbin thread and the red dot for Janome thread.
Thank you for your question regarding the yellow bobbin case. While it is stated the yellow bobbin case is for non-Janome thread, it is actually made for thinner thread that needs higher tension. Janome brand prewound bobbin thread is a thinner weight thread (60wt) so you would use the yellow bobbin case with the Janome prewound thread.