Monofiliment thread comes in different sizes (as opposed to weights) when it is nylon. The only exception I know of is Superior's Monopoly which is polyester and is designated as 100 wt which is the equivelent of a silk thread. This website helps explain the different sizes. https://www.thethreadexchange.com/miva/merchant.mvc?Screen=CTGY&Category_Code=monofilament-information
Personally I prefer the super fine monofilament but I only use it for ditching. I have yet to do all of my quilting with a mono and I probably never will, opting instead for a super fine thread like Invisifil by Wonderfil, Micro quilter by Superior or silk but I find the price of silk expensive so I reserve it for hand applique. I would not use monofilament in the bobbin. Use a fine thread like Decobob (by wonderful) in the bobbin. When I quilt with mono I usually use a 60 wt in the bobbin but I use a longarm.
A super fine thread like the 3 I have mentioned above, in a neutral color like a sage green, gold or gray will not detract from rainbow colored blocks. It sinks in and blends.