Set Vim colorscheme by time of day, an example from book Learning Vi and Vim.

SetTimeOfDayColors.vim

function SetTimeOfDayColors()
    let g:Favcolorschemes = ["darkblue", "morning", "shine", "evening"]
    let g:CurrentHour = (strftime("%H") + 0) / 6
    if g:colors_scheme !~ g:Favcolorschemes[g:CurrentHour]
        execute "colorscheme " . g:Favcolorschemes[g:CurrentHour]
        redraw
    endif
endfunction