hi Lorna, sorry for a late response, i’ve been neglecting orchid
i used to work for a firm in the uk that specialised in bi-colour
without getting technical on different ways to approach the issue
(although if anyone requests i would be more than happy to share),
the easiest way without a ring stretcher, would be to use a solid
centre band with two white gold edges.—>if your total width is
6mm, then make two white gold bands 1.2mm leaving room to file and
finish and make your centre 4 mm (adjust according to style!).
round up your rings slightly deeper than required and slightly
smaller than the needed size to allow room for discrepancies. tie
all 3 with some binding wire and fuse/solder together.
for a cleaner edge you could flood some solder on to the side of the
centre ring and sand most of it off before soldering to the edges.
that way you would have less clean up on the top of the ring.
dont forget most commercial bi-coloured rings are finished
(grooved/mill-grained) with a lathe, although its possible to do a
more than adequate job by hand with a saw and needle files.
good luck, jon