The right way to override any highlighting if you don't want to edit the colorscheme file directly

colorscheme-override.md

Suppose you have weird taste and you absolutely want:

• your visual selection to always have a green background and black foreground,
• your active statusline to always have a white background and red foreground,
• your very own deep blue background.

Your first reflex is probably to put those lines somewhere in your vimrc:

highlight Visual cterm=NONE ctermbg=76 ctermfg=16 gui=NONE guibg=#5fd700 guifg=#000000
highlight StatusLine cterm=NONE ctermbg=231 ctermfg=160 gui=NONE guibg=#ffffff guifg=#d70000
highlight Normal cterm=NONE ctermbg=17 gui=NONE guibg=#00005f
highlight NonText cterm=NONE ctermbg=17 gui=NONE guibg=#00005f

But it quickly appears that those lines have no effect if you source a colorscheme later in your vimrc so you move them below:

colorscheme foobar
highlight Visual cterm=NONE ctermbg=76 ctermfg=16 gui=NONE guibg=#5fd700 guifg=#000000
highlight StatusLine cterm=NONE ctermbg=231 ctermfg=160 gui=NONE guibg=#ffffff guifg=#d70000
highlight Normal cterm=NONE ctermbg=17 gui=NONE guibg=#00005f
highlight NonText cterm=NONE ctermbg=17 gui=NONE guibg=#00005f

which gives the desired outcome. Everything is fine. In principle.

But you like to try new colorschemes, or you prefer to have different colorschemes for different filetypes or time of the day and your overrides don't carry over when you change your colorscheme.

This is because colorschemes usually reset all highlighting, including your own, when they are sourced.

The solution is to override the desired highlights in an autocommand that's executed whenever a colorscheme is sourced:

augroup MyColors
    autocmd!
    autocmd ColorScheme * highlight Visual cterm=NONE ctermbg=76 ctermfg=16 gui=NONE guibg=#5fd700 guifg=#000000
                      \ | highlight StatusLine cterm=NONE ctermbg=231 ctermfg=160 gui=NONE guibg=#ffffff guifg=#d70000
                      \ | highlight Normal cterm=NONE ctermbg=17 gui=NONE guibg=#00005f
                      \ | highlight NonText cterm=NONE ctermbg=17 gui=NONE guibg=#00005f
augroup END
colorscheme foobar

Note that the autocmd must be added before any colorscheme is sourced or it will have no effect.

Of course, we can go one step further and put all our highlights in a neat little function for maximum readability/managability/maintainability:

function! MyHighlights() abort
    highlight Visual     cterm=NONE ctermbg=76  ctermfg=16  gui=NONE guibg=#5fd700 guifg=#000000
    highlight StatusLine cterm=NONE ctermbg=231 ctermfg=160 gui=NONE guibg=#ffffff guifg=#d70000
    highlight Normal     cterm=NONE ctermbg=17              gui=NONE guibg=#00005f
    highlight NonText    cterm=NONE ctermbg=17              gui=NONE guibg=#00005f
endfunction

augroup MyColors
    autocmd!
    autocmd ColorScheme * call MyHighlights()
augroup END
colorscheme foobar

NOTE: This won't work if you have an autocommand like the following in your vimrc because autocommands don't nest by default:

[...]
autocmd BufWritePost $MYVIMRC source $MYVIMRC
[...]

The solution is to add the nested keyword to that autocommand to allow other autocommands to be triggered by that event:

[...]
autocmd BufWritePost $MYVIMRC nested source $MYVIMRC
[...]