What defines default colors for highlight groups not present in a color scheme?

Question

After updating to vim 9.1 yesterday, diff highlighting colors were broken for me. Comparing with an older vim (7.4) on a different machine, it seems that the cause was probably a change in the diff.vim syntax file using DiffAdd/Delete/Change as the relevant highlighting groups instead of Identifier/Special/PreProc like previously; thus I fixed this by adding the appropriate hi entries to my color scheme. But during the investigation, I could not figure out how the default colors for those hl groups are determined.

For example, here are the values for DiffDelete as given by :echo hlget('DiffDelete') before adding a hi DiffDelete entry to my scheme:

[{'id': 30, 'ctermfg': '12', 'name': 'DiffDelete', 'gui': {'bold': v:true},
'term': {'bold': v:true}, 'ctermbg': '6', 'guibg': 'DarkCyan', 'guifg': 'Blue'}]

Also, vim used the guifg/bg values instead of the ctermfg/bg values, as ctermfg=12 should be red but deleted lines were highlighted blue on cyan.

Example:

.

I checked the following places for highlighting definitions and related config:

• The default color scheme under $VIMRUNTIME/colors/default.vim is basically empty and does not define any relevant colors.
• My existing color scheme (based on mustang) did not define them either.
• Other color schemes in $VIMRUNTIME/colors definie DiffDelete highlights, but no scheme matches even one of the fg/bg values.

I'm setting t_Co=256 in my vimrc and $TERM is xterm-256color; I don't think any other vimrc settings are relevant.

So in summary, here's my questions:

• Are the default color values such as the hlget output above hardcoded somewhere in the vim code? If not, is it in some configuration file outside of $VIMRUNTIME, or is it dynamically determined based on the env / terminal / whatever?
• Why does the default highlighting above result in the guibg/fg being applied instead of the ctermbg/fg values - shouldn't the gui values only be used in gvim etc and not in a terminal?

Answer 1

Are the default color values such as the hlget output above hardcoded somewhere in the vim code?

Yes, the defaults are hardcoded in C source. Except, named color constants are defined in VimScript runtime (in late builds). That would be a shame if Vim crashed on startup if one had erased VIMRUNTIME folder, wouldn't it?

Why does the default highlighting above result in the guibg/fg being applied instead of the ctermbg/fg values

This depends on :h 'termguicolors' option setting. It is off by default, so blame your config. However, many popular color schemes do not provide any reasonable values for 256 color palette, and almost all terminal programs have support for TrueColor anyway. So everyone types set termguicolors into personal vimrc, and only few bother about its meaning.

Answer 2

FWIW, here is the content of a diff file without the change mentioned by Christian in the comments:

Obviously, the exact colors depend on the colorscheme, but it certainly is meaningful and readable.

Now, here is how the same file looks after the change in question:

This is way too noisy.