You could sew two different ribbons together and set a grommet in the seam. That's basically how the original 4-color ribbon is. Even with the original ribbon you had to wind the ribbon to the middle and undo the ribbon from the vibrator and swap the side the middle grommet is on to change the color.
Via Ted Munk at https://typewriterdatabase.com/1969-sears-medalist-power-12.24661.typewriter
A furtherrefinementinmaking two-colourribbonsisthe useofacentreliningmachine, which automatically drawsathininsulatinglineon theribbon.A transparentgumsolutionispumpedbyairpressure througha stainless needle whichpreventsthecolours from running.
Center lining machines are used to make bichrome typewriter ribbon. It operates by drawing an insulating line on the ribbon and transparent gum solution is pumped through a needle using pneumatic pressure to keep the two colors from running into each other.
Part 4: COMPLETE Olympia SM3 Service and Repair Series: STICKY RIBBON LIFTER by [[The HotRod Typewriter Co.]]
The universal bar lifts the ribbon vibrator.
Adjustment points to adjust the ribbon lift heights for issues with red/black on bichrome use.
Maximum travel of the universal bar adjustment screws on crossbar that attaches to springs. timestamp 5:29
Screws at the ends of the cross bar which are attached to the key springs can be adjusted slightly to provide for heavier or lighter touch control. Timestamp 6:07