DIY Range of color with terminal emulators

[ This is based on partial information. Please help verify and/or expand these findings. ]

When Emacs runs in a non-windowed session its color reproduction capacity is framed or determined by the underlying terminal emulator (More accurate colors in terminal emulators). Emacs cannot produce a color that lies outside the range of what the terminal’s color palette renders possible.

This is immediately noticeable when the terminal’s first 16 codes do not include a pure black value for the ‘termcol0’ entry and a pure white for ‘termcol15’. Emacs cannot set the correct background (white for modus-operandi; black for modus-vivendi) or foreground (inverse of the background). It thus falls back to the closest approximation, which seldom is appropriate for the purposes of the Modus themes.

In such a case, the user is expected to update their terminal’s color palette such as by adapting these resources (Preview theme colors):

! Theme: modus-operandi
! Description: XTerm port of modus-operandi (Modus themes for GNU Emacs)
! Author: Protesilaos Stavrou, <https://protesilaos.com>
xterm*background: #ffffff
xterm*foreground: #000000
xterm*color0:     #000000
xterm*color1:     #a60000
xterm*color2:     #005e00
xterm*color3:     #813e00
xterm*color4:     #0031a9
xterm*color5:     #721045
xterm*color6:     #00538b
xterm*color7:     #bfbfbf
xterm*color8:     #595959
xterm*color9:     #972500
xterm*color10:    #315b00
xterm*color11:    #70480f
xterm*color12:    #2544bb
xterm*color13:    #5317ac
xterm*color14:    #005a5f
xterm*color15:    #ffffff

! Theme: modus-vivendi
! Description: XTerm port of modus-vivendi (Modus themes for GNU Emacs)
! Author: Protesilaos Stavrou, <https://protesilaos.com>
xterm*background: #000000
xterm*foreground: #ffffff
xterm*color0:     #000000
xterm*color1:     #ff8059
xterm*color2:     #44bc44
xterm*color3:     #d0bc00
xterm*color4:     #2fafff
xterm*color5:     #feacd0
xterm*color6:     #00d3d0
xterm*color7:     #bfbfbf
xterm*color8:     #595959
xterm*color9:     #ef8b50
xterm*color10:    #70b900
xterm*color11:    #c0c530
xterm*color12:    #79a8ff
xterm*color13:    #b6a0ff
xterm*color14:    #6ae4b9
xterm*color15:    #ffffff