I like to consider myself an expert on sparkly jazzmasters.
If you're still thinking, Shoreline over Inca. Of my two Jagmasters, one kept it's original bright silver, the other aged to gold - and the gold looks much better with the stock black guard than the silver one does. Since the silver one is the "modding" one anyway ("gold" has emotional ties) when I needed a new guard I went for a Jaguar-style tort instead of a replacement black.
Since black is your key-part-to-keep, gold should be the accent; at least according to my taste.
By the way, I think the "correct" answer is either to live with it as is; or to overlay a thin veneer and sand it back to shape afterward. If the edge is too rounded over for the give in a veneer to clamp into, then you're talking routing a small section away and cutting a plug to fit that you can then sand a "new" shape into; or (if you wanted to keep the door open for future sunburst) planing the entire back of the guitar down and adding a few mm veneer/cap across the whole back. The plug is probably the riskiest option in terms of witness marks reappearing down the line though. so this approach may be better even for a solid finish, though the obvious downside is that it removes even more original wood.
- they won't sell the covers alone though. ;-)
