I created a new palette, but is there a way to check what palette colors are missing?
In haste, without expertise: In the control panel appearence pane, if you’re looking at the bottom, you can check the box:
Show color names that are not part of the current palette
as one step in troubleshooting.
I’m not sure whether what’s “missing” is standardized. It may just show you color names that exist in other palettes but not yours (which doesn’t mean they’re essential).
EDIT: seems important to point to Palette Manager — Preview
as a great tool in palette development (perhaps you’re already using it?)
… and to suggest that the question of what’s “missing” is necessarily complex, because reverting to the default html color is a problem only when the contrast matters in context (light theme? dark theme? Surprising sidebar or alert backgrounds that require their own foregrounds?), which may not be an objective or project-neutral judgment.
Whenever I make palettes and I want to look and see what is missing, I often take my palette, and the default vanilla palette and throw them into a diff checker website to compare what is there and isn’t, since some palettes have additional non-default colors, like base 001 etc.
Tiddlywiki has difference tools that may help here too.