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";
}'

In 24-bit-truecolor.sh the parts that use seq to count down (seq 255 128) work on BSD seq but fail on GNU seq so if you run the script on Linux it only prints half the lines. However, you can use an alternate syntax that works on both: seq 255 -1 128