Tracked this down to a change in wxWidgets:
https://github.com/wxWidgets/wxWidgets/commit/24dbe9775c1901b24a514ead28923227922b30af
They moved from a generic RGB color space to an SRGB color space. Thus, colors are getting mapped funky. It's trivial to rebuild wxWidgets with the old color space, but then parts of the UI (particularly dark mode things) don't look correct in Mojave as Mojave uses SRGB for display on any monitor that can handle it. (any recent Mac)