Ansi color cube with 24 grays (240 colors), and a simple 3x3x3 cube with 5-value grayscale ramp (32 colors).

colors-256.fish

#!/usr/bin/env fish

set -l G1 (seq 232 243)
set -l G2 231 15 (seq 255 244)
set -l G3 (seq 244 255) 231

switch "$argv[1]"
   case '-h' '--help'
      echo 'USAGE: colors-256 [-v|--verbose]' >&2
      exit

   case '-v' '--verbose'
echo 'col % go    \e[%G'\t'move line,col \e[%;%Hm'\t%ABCD for udrl >&2
echo 'reset pos   \e[Hm'\tclear cur \\e\[%Xm\tdel cur \\e\[%Pm >&2
echo reset attrs \\e\[0m\tclear all \\e\[Jm\t\tclear line \\e\[Km >&2
echo set attrs '  \e[*m  \e[*;*m  \e[*;*;*m' >&2
echo fore \\e\[30-37m\t back \\e\[40-47m\t'    'xterm-fore \\e\[38\;5\;%m'   'xterm-back \\e\[48\;5\;%m >&2
echo fore \\e\[90-97m\t back \\e\[100-107m\treset-fore \\e\[39m\treset-back \\e\[49m >&2
echo -e attrs\t \e\[1m1\e\[0m bold\t\e\[2m2\e\[0m dim\t\e\[3m3\e\[0m slant\t\e\[4m4\e\[0m under\t5 blink\t\e\[7m7\e\[0m inv\t8 n/a\t\e\[9m9\e\[0m strike >&2
set N '+'
echo >&2

for X in 16 0 $G1 $G2
   if test 244 -gt "$X"
      switch "$X"
         case 231 15
      if test 231 -eq "$X"
         echo -ne \n'\e[38;5;16m\e[48;5;'$X'm' $X \e\[m
      else
         echo -ne '\e[38;5;16m\e[48;5;'$X'm \e[2m?\e[0m\e[38;5;16m\e[48;5;'$X'm'$X \e\[m
      end
         case 0
      echo -ne '\e[38;5;15m\e[48;5;'$X'm  \e[2m?\e[0m\e[38;5;15m\e[48;5;'$X'm'$X \e\[m
         case 16
      echo -ne '\e[38;5;15m\e[48;5;'$X'm  '$X \e\[m
         case '*'
         echo -ne '\e[38;5;15m\e[48;5;'$X'm' $X \e\[m
      end #switch
   else
      echo -ne '\e[38;5;16m\e[48;5;'$X'm' $X \e\[m
   end
end
echo \n

set -l V3_1  16  18  21  -  28  32 39   46  49  51
set -l V3_2 124  90  93  - 136 139 141  154 157 195
set -l V3_3 196 198 201  - 208 211 219  226 229 231
echo -en "\e[5m"

set -l FG 231
for X in $V3_1
   if test "+-" = "+$X"
      set FG 0
      continue
   end
   echo -ne '\e[38;5;'$FG'm\e[48;5;'$X'm '$X' \e[49m'
end
echo
for X in $V3_1
   if test "+-" = "+$X"
      continue
   end
   echo -ne '\e[48;5;'$X'm    \e[49m'
end
echo
set -l FG 231
for X in $V3_2
   if test "+-" = "+$X"
      set FG 0
      continue
   end
   if test 2 -eq (string length -- "$X")
      echo -ne '\e[38;5;'$FG'm\e[48;5;'$X'm '$X' \e[49m'
   else
      echo -ne '\e[38;5;'$FG'm\e[48;5;'$X'm'$X' \e[49m'
   end
end
echo
for X in $V3_2
   if test "+-" = "+$X"
      continue
   end
   echo -ne '\e[48;5;'$X'm    \e[49m'
end
echo
set -l FG 231
for X in $V3_3
   if test "+-" = "+$X"
      set FG 16
      continue
   end
   echo -ne '\e[38;5;'$FG'm\e[48;5;'$X'm'$X' \e[49m'
end
echo
for X in $V3_3
   if test "+-" = "+$X"
      continue
   end
   echo -ne '\e[48;5;'$X'm    \e[49m'
end
echo -e '\e[0m'

   case '*'
end #switch

set -l C (seq 16 231)
set -l P (seq 0 7)
set -l Q (seq 8 15)
set -l W 16 196 46 226 21 201 51 231

if test -z "$N"
   for X in 16 $G1 $G3
      echo -ne '\e[48;5;'$X'm  \e[49m'
   end
else
   echo
   for X in 16 234 238 244
      if test 2 -eq (string length -- "$X")
         echo -ne '\e[38;5;231m\e[48;5;'$X'm ' $X \e\[m
      else
         echo -ne '\e[38;5;231m\e[48;5;'$X'm '$X \e\[m
      end
   end #for
   for X in 249 254 231
      echo -ne '\e[38;5;16m\e[48;5;'$X'm' $X \e\[m
   end #for
   echo
end
echo

for X in $C
   echo -ne '\e[48;5;'$X'm  \e[49m'
   if test 0 -eq (math -s0 "($X -15) % 36")
      if test -n "$N"
         echo -n ' '(math -s0 "$X -35")-$X
      end
      echo
   end
end
echo

for X in $P
   if test -z "$N"
      echo -ne '\e[97;4'$X'm  \e[49m'
   else
      echo -ne '\e[97;4'$X'm ' $X ' \e[m'
   end
end
echo

for X in $W
   if test -z "$N"
      echo -ne '\e[38;5;231m\e[48;5;'$X'm  \e[49m'
   else
      if test 2 -eq (string length -- "$X")
         echo -ne '\e[38;5;231m\e[48;5;'$X'm ' $X' \e[m'
      else
         echo -ne '\e[38;5;16m\e[48;5;'$X'm '$X '\e[m'
      end
   end
end
echo

for X in $Q
   if test -z "$N"
      echo -ne '\e[48;5;'$X'm  \e[49m'
   else
      if test 9 -ge "$X"
         echo -ne '\e[38;5;16m\e[48;5;'$X'm ' $X ' \e[m'
      else
         echo -ne '\e[38;5;16m\e[48;5;'$X'm '$X ' \e[m'
      end
   end
end
echo

asciinema.org/a/438882
License MIT, free to do whatever. Yours to fuck up.

Test images imgur.com/gallery/6HOORem

Photography test imgur.com/gallery/6pD6coY