Thanks for the comments so far, everyone.
Since posing the question, I have thought to browse the Internet for pictures of the 'Black Diamond Edition' of the CY 328, which were produced by Cruisers Yachts in the 2014 time frame, the introductory year of the boat (see below). This is the 'look' that I am aiming towards with a few slight variations.
I have not found very many good resolution pictures of the red backrest sections, but of what I've found, it looks like there was a combination of red and black thread used in the red sections.
- RED for the border stitching of the red panels and
- BLACK for the french seam across the top.
I may go with this approach since it would match the black stitching of the gray vinyl where it needed to and blend with the red vinyl elsewhere.
Having so many colors sure gets complicated.