Jeroen Bakker told me to bump the subversion.
I moved all the "Versioning code until next subversion bump goes here." code into a version check block in versioning_300.cc, and adjusted my if block in versioning_userdef.c.
This is something we missed in #104599. Without it, the default color isn't always set.
Thanks to user roman-13 for pointing out the problem, and thanks to Hans Goudey for telling me the solution.