This Norwegian knitting thimble (or ring) can be very useful tool if you are working with two colours and like to hold your yarn in your left hand.

The same rules apply for the thimble as with using two colours, the yarn further away will need to travel further making smaller stitches. This makes it best for the background colour.

I would encourage you to experiment to see which location works best for you to see if this tool helps make colourwork easier for you to work.