test if your terminal supports 24 bit truecolor

24-bit-truecolor.sh

#!/bin/bash
#
#   This file echoes four gradients with 24-bit color codes
#   to the terminal to demonstrate their functionality.
#   The foreground escape sequence is ^[38;2;<r>;<g>;<b>m
#   The background escape sequence is ^[48;2;<r>;<g>;<b>m
#   <r> <g> <b> range from 0 to 255 inclusive.
#   The escape sequence ^[0m returns output to default

SEPARATOR=':'

setBackgroundColor()
{
    echo -en "\x1b[48${SEPARATOR}2${SEPARATOR}$1${SEPARATOR}$2${SEPARATOR}$3""m"
}

resetOutput()
{
    echo -en "\x1b[0m\n"
}

# Gives a color $1/255 % along HSV
# Who knows what happens when $1 is outside 0-255
# Echoes "$red $green $blue" where
# $red $green and $blue are integers
# ranging between 0 and 255 inclusive
rainbowColor()
{ 
    let h=$1/43
    let f=$1-43*$h
    let t=$f*255/43
    let q=255-t

    if [ $h -eq 0 ]
    then
        echo "255 $t 0"
    elif [ $h -eq 1 ]
    then
        echo "$q 255 0"
    elif [ $h -eq 2 ]
    then
        echo "0 255 $t"
    elif [ $h -eq 3 ]
    then
        echo "0 $q 255"
    elif [ $h -eq 4 ]
    then
        echo "$t 0 255"
    elif [ $h -eq 5 ]
    then
        echo "255 0 $q"
    else
        # execution should never reach here
        echo "0 0 0"
    fi
}

for i in `seq 0 127`; do
    setBackgroundColor $i 0 0
    echo -en " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor $i 0 0
    echo -en " "
done
resetOutput

for i in `seq 0 127`; do
    setBackgroundColor 0 $i 0
    echo -n " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor 0 $i 0
    echo -n " "
done
resetOutput

for i in `seq 0 127`; do
    setBackgroundColor 0 0 $i
    echo -n " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor 0 0 $i
    echo -n " "
done
resetOutput

for i in `seq 0 127`; do
    setBackgroundColor `rainbowColor $i`
    echo -n " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor `rainbowColor $i`
    echo -n " "
done
resetOutput

README.md

# shell script
printf "\x1b[38;2;255;100;0mTRUECOLOR\x1b[0m\n"

If your terminal emulator does NOT display the word TRUECOLOR in red, it does not support 24-bit color. If you don't want to switch to a different terminal emulator that supports 24-bit color, proceed to step 3. (After installation, the g:onedark_termcolors option may interest you.)

If your terminal emulator displays the word TRUECOLOR in red, it supports 24-bit color, and you should add the following lines to your ~/.vimrc to enable 24-bit color terminal support inside Vim.

"Use 24-bit (true-color) mode in Vim/Neovim when outside tmux.
"If you're using tmux version 2.2 or later, you can remove the outermost $TMUX check and use tmux's 24-bit color support
"(see < http://sunaku.github.io/tmux-24bit-color.html#usage > for more information.)
if (empty($TMUX))
  if (has("nvim"))
    "For Neovim 0.1.3 and 0.1.4 < https://github.com/neovim/neovim/pull/2198 >
    let $NVIM_TUI_ENABLE_TRUE_COLOR=1
  endif
  "For Neovim > 0.1.5 and Vim > patch 7.4.1799 < https://github.com/vim/vim/commit/61be73bb0f965a895bfb064ea3e55476ac175162 >
  "Based on Vim patch 7.4.1770 (`guicolors` option) < https://github.com/vim/vim/commit/8a633e3427b47286869aa4b96f2bfc1fe65b25cd >
  " < https://github.com/neovim/neovim/wiki/Following-HEAD#20160511 >
  if (has("termguicolors"))
    set termguicolors
  endif
endif

Reference

https://vimawesome.com/plugin/onedark-vim

true-color.sh

#!/bin/bash
awk 'BEGIN{
    s="/\\/\\/\\/\\/\\"; s=s s s s s s s s;
    for (colnum = 0; colnum<77; colnum++) {
        r = 255-(colnum*255/76);
        g = (colnum*510/76);
        b = (colnum*255/76);
        if (g>255) g = 510-g;
        printf "\033[48;2;%d;%d;%dm", r,g,b;
        printf "\033[38;2;%d;%d;%dm", 255-r,255-g,255-b;
        printf "%s\033[0m", substr(s,colnum+1,1);
    }
    printf "\n";
}'