Piers, I think you are correct in that the Japanese preference for the sun is red, much as the moon is generally seen in silver. It seems as with so many things in this hobby nothing is set in stone. Obviously there were additional colors seen in their representations of the sun, such as gold and others. Likely a variety of reasons. It is a very precise and time consuming process to produce that deep red seen on some kodogu.
Viewing your tsuba, do you think the insert was once plated with gold or silver or do you think the copper was the original look? Looking at it enlarged I can't tell for certain, but it seems to show traces of a light color (silver?) at the top and bottom. Perhaps it was originally depicting the moon. ???
I think the gold plug on my kozuka was to give it a little bling. I would have preferred something a bit more subtle, but......