Oh RJ, I just went and peeked at your Christmas block. Wow, what a lot of work that must have been. If I invested that much time and effort into the piecing I would do the same for the quilting. I would change thread colors to match where you want it, rather than trying to find one color to do the whole thing. The beauty of silk and 100wt is that you don't have to be exact. You could use one shade of green for all the different greens, same for the blues, reds, etc.
This project would also allow you the opportunity to embellish with threads like using a metallic on the Christmas tree garland, star and the candle flame.
But if your intent is to ditch the main elements and leave the quilting at that, then I think you will do fine with either an invisible monofilament or a neutral color of a 100wt like Invisifil or silk. I hope you can find it in the UK.