Four strand guilloche (a quick way to work out is to count the single white tesserae on a 45 degree line then add 1, that's how many strands).
I've put this up to show how they've set the colours. You can
use the same colour for each strand following it as it goes under and over the others, or as here, you can have the same colours for the inside, the middle and the outside.
Image copyright Lawrence Payne