Re: 3 different bobbin holders
Yellow - embroidery - higher tension - use when using thiner bobbin fil thread than Janome bobbin thread, or when you need to tighten the bottom tension for other reasons - eg using a fine thread, or wanting to pull the top thread down when doing decorative stitches, if the red dot case doesn't work (but try the red dot first)
Red - standard - tension normal - use when using Janome bobbin thread or when you want a mid range of tension such as in normal sewing mode, and not using speciality threads
Blue - quilting - lower tension - use when quilting through thick layers, or when you want to lower the bottom tension for certain threads - maybe when using a thicker thread in the bottom than the top.
I think about tension like a corridor - a narrow corridor ( high tension) lets fewer people (thinner thread through) - if you use a thicker thread (more people) it will be held back because it can't get through the 'narrow corridor' so in effect increasing the tension on the thread. Use a lower tension case (wider corridor) to let the thicker thread through at the same tension as a thinner thread with a higher tension case. Hope that makes sense!
"Life may not be the party we hoped for, but while we're here we should dance". Anon