A poor man's previewer for Vim's colorscheme

preview-colorscheme.vim

" Usage: :source this file.

let s:file = expand('<sfile>')

function! s:go()
  let inData = 0
  for line in readfile(s:file)
    if inData && line =~# 'xxx'
      let name = split(line)[0]
      execute printf('syntax match test%s /^%s .*/', name, name)
      execute printf('highlight default link test%s %s', name, name)
    else
      if line ==# 'finish'
        let inData = !0
      endif
    endif
  endfor
endfunction

call s:go()

finish
-------------------------------------------------------------------------------
Boolean        xxx links to Constant
Character      xxx links to Constant
ColorColumn    xxx term=reverse ctermbg=4 guibg=DarkRed
Comment        xxx term=bold ctermfg=3 guifg=SkyBlue
Conceal        xxx ctermfg=7 ctermbg=8 guifg=LightGrey guibg=DarkGrey
Conditional    xxx links to Statement
Constant       xxx term=underline ctermfg=6 guifg=#ffa0a0
Cursor         xxx guifg=slategrey guibg=khaki
CursorColumn   xxx term=reverse ctermbg=8 guibg=Grey40
CursorIM       xxx cleared
CursorLine     xxx term=underline cterm=underline guibg=Grey40
Debug          xxx links to Special
Define         xxx links to PreProc
Delimiter      xxx links to Special
DiffAdd        xxx term=bold ctermbg=4 guibg=DarkBlue
DiffChange     xxx term=bold ctermbg=5 guibg=DarkMagenta
DiffDelete     xxx term=bold cterm=bold ctermfg=4 ctermbg=6 gui=bold guifg=Blue guibg=DarkCyan
DiffText       xxx term=reverse cterm=bold ctermbg=1 gui=bold guibg=Red
Directory      xxx term=bold ctermfg=3 guifg=Cyan
Error          xxx term=reverse cterm=bold ctermfg=7 ctermbg=1 guifg=White guibg=Red
ErrorMsg       xxx term=standout cterm=bold ctermfg=7 ctermbg=1 guifg=White guibg=Red
Exception      xxx links to Statement
Float          xxx links to Number
FoldColumn     xxx term=standout ctermfg=8 guifg=tan guibg=grey30
Folded         xxx term=standout ctermfg=8 guifg=gold guibg=grey30
Function       xxx links to Identifier
Identifier     xxx term=underline cterm=bold ctermfg=6 guifg=palegreen
Ignore         xxx cterm=bold ctermfg=8 guifg=grey40
IncSearch      xxx term=reverse ctermfg=14 ctermbg=10 gui=reverse guifg=slategrey guibg=khaki
Include        xxx links to PreProc
Keyword        xxx links to Statement
Label          xxx links to Statement
LineNr         xxx term=underline ctermfg=3 guifg=Yellow
Macro          xxx links to PreProc
MatchParen     xxx term=reverse ctermbg=3 guibg=DarkCyan
ModeMsg        xxx term=bold ctermfg=6 gui=bold guifg=goldenrod
MoreMsg        xxx term=bold ctermfg=2 gui=bold guifg=SeaGreen
NonText        xxx term=bold cterm=bold ctermfg=1 gui=bold guifg=LightBlue guibg=grey30
Normal         xxx guifg=White guibg=grey20
Number         xxx links to Constant
Operator       xxx links to Statement
Pmenu          xxx cterm=underline guifg=White guibg=grey20
PmenuSbar      xxx cterm=reverse guifg=White guibg=grey20
PmenuSel       xxx cterm=underline guifg=wheat guibg=peru
PmenuThumb     xxx guifg=wheat guibg=peru
PreCondit      xxx links to PreProc
PreProc        xxx term=underline ctermfg=5 guifg=indianred
Question       xxx term=standout ctermfg=10 gui=bold guifg=springgreen
Repeat         xxx links to Statement
Search         xxx term=reverse ctermfg=7 ctermbg=9 guifg=wheat guibg=peru
SignColumn     xxx term=standout ctermfg=11 ctermbg=8 guifg=Cyan guibg=Grey
Special        xxx term=bold ctermfg=5 guifg=navajowhite
SpecialChar    xxx links to Special
SpecialComment xxx links to Special
SpecialKey     xxx term=bold ctermfg=2 guifg=yellowgreen
SpellBad       xxx term=reverse ctermbg=12 gui=undercurl guisp=Red
SpellCap       xxx term=reverse ctermbg=9 gui=undercurl guisp=Blue
SpellLocal     xxx term=underline ctermbg=11 gui=undercurl guisp=Cyan
SpellRare      xxx term=reverse ctermbg=13 gui=undercurl guisp=Magenta
Statement      xxx term=bold ctermfg=3 gui=bold guifg=khaki
StatusLine     xxx term=bold,reverse cterm=bold,reverse guifg=black guibg=#c2bfa5
StatusLineNC   xxx term=reverse cterm=reverse guifg=grey50 guibg=#c2bfa5
StorageClass   xxx links to Type
String         xxx links to Constant
Structure      xxx links to Type
TabLine        xxx term=reverse ctermfg=7 ctermbg=8 gui=underline guibg=DarkGrey
TabLineFill    xxx term=reverse ctermfg=7 ctermbg=8 gui=reverse
TabLineSel     xxx term=bold,reverse cterm=bold,underline ctermfg=7 ctermbg=8 gui=bold
Tag            xxx links to Special
Title          xxx term=bold ctermfg=5 gui=bold guifg=indianred
Todo           xxx term=standout ctermfg=0 ctermbg=14 guifg=orangered guibg=yellow2
Type           xxx term=underline ctermfg=2 gui=bold guifg=darkkhaki
Typedef        xxx links to Type
Underlined     xxx term=underline cterm=underline ctermfg=5 gui=underline guifg=#80a0ff
VertSplit      xxx term=reverse cterm=reverse guifg=grey50 guibg=#c2bfa5
Visual         xxx term=reverse cterm=reverse ctermbg=8 guifg=khaki guibg=olivedrab
VisualNOS      xxx term=bold,underline cterm=bold,underline gui=bold,underline
WarningMsg     xxx term=standout ctermfg=1 guifg=salmon
WildMenu       xxx term=standout ctermfg=0 ctermbg=3 guifg=Black guibg=Yellow
-------------------------------------------------------------------------------

Why don't you use something like nnoremap <leader>hi :source $VIMRUNTIME/syntax/hitest.vim<CR>? It works perfectly for me.

Because I dislike it. It has several side effects, and it might not work depending on currently defined key mappings.