Now more terminals bring support for 16 million colors (see references below).

Here's a test case for terminal:

printf "\x1b[${bg};2;${red};${green};${blue}m\n"

The 256 colour palete is configured at start, and it's a 666 cube of colours, each of them defined as a 24bit (888 rgb) colour.

This means that current support can only display 256 different colours in the terminal, while truecolour means that you can display 16 milion different colours at the same time.

Truecolour escape codes doesnt uses a colour palete. It just specifies the colour itself.

Here's a test case:

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

or https://raw.githubusercontent.com/JohnMorales/dotfiles/master/colors/24-bit-color.sh
or http://github.com/robertknight/konsole/tree/master/tests/color-spaces.pl
or https://git.gnome.org/browse/vte/tree/perf/img.sh?h=vte-0-36
or just run this:

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";
}'

Keep in mind that it is possible to use both ';' and ':' as parameters delimiter.

According to Wikipedia[1], this is only supported by xterm and konsole.

[1] https://en.wikipedia.org/wiki/ANSI_color

Currently, there is no support for the 24-bit colour descriptions in the terminfo/termcap database and utilites. See the discussion thread here: https://lists.gnu.org/archive/html/bug-ncurses/2013-10/msg00007.html

Here are terminals discussions:

Now supporting truecolour

st (from suckless) [delimeter: semicolon] - http://lists.suckless.org/dev/1307/16688.html
konsole [delimeter: colon, semicolon] - https://bugs.kde.org/show_bug.cgi?id=107487
iterm2 [delimeter: colon, semicolon] - since v3 version
qterminal [delimeter: semicolon] - https://github.com/qterminal/qterminal/issues/78
cool-retro-term [delimeter: semicolon]
Black Screen [delimeter: semicolon] - crossplatform, HTML/CSS/JS-based
Tera Term [delimeter: colon, semicolon] - Windows platform
ConEmu [delimeter: semicolon] - Windows platform
FinalTerm [delimeter: semicolon] - abandoned, iTerm2 borrowing it's ideas and features.
MacTerm [delimeter: semicolon] - Mac OS X platform
mintty [delimeter: semicolon] Cygwin and MSYS/MSYS2 since commit https://github.com/mintty/mintty/commit/43f0ed8a46c6549cb9a3ea27abc057b5abe13bdb (2.0.1 release) - Windows platform
all libvte based terminals (since 0.36 version) [delimeter: colon, semilocon] - https://bugzilla.gnome.org/show_bug.cgi?id=704449
- libvte-based Gnome Terminal
- libvte-based sakura
- libvte-based xfce4-terminal - since 0.6.90 release, if compiled with GTK+3
- libvte-based Terminator - use GTK+3 version.
- libvte-based Terminix - written in D. Similar user interface as for Terminator.
- libvte-based Lilyterm - since commit https://github.com/Tetralet/LilyTerm/commit/72536e7ba448ad9ef1126ce45fbde3a3407a271b
- libvte-based ROXTerm
- libvte-based evilvte - no release yet, version from git https://github.com/caleb-/evilvte
- libvte-based Termit
- libvte-based Termite
- libvte-based Tilda
- libvte-based tinyterm
- libvte-based lxterminal - with --enable-gtk3 configure flag.
- libvte-based mlterm - with --with-gtk=3.0 configure flag

But there are bunch of libvte-based terminals for GTK2 so they are listed in the another section.

Also, while this one is not exactly a terminal, but a terminal replayer, it still worth mentioning:

asciinema player: https://github.com/asciinema/asciinema-player

Parsing ANSI colour sequences, but approximating them to 256 palette

xterm (though doing it wrong: "it uses nearest colour in RGB colour space, with a usualfalse assumption about orthogonal axes")
urxvt aka rxvt-unicode - since Revision 1.570 http://lists.schmorp.de/pipermail/rxvt-unicode/2016q2/002261.html
linux console (since v3.16): https://github.com/torvalds/linux/commit/cec5b2a97a11ade56a701e83044d0a2a984c67b4
Windows 10 bash console, since build 14352, approximates 256 and 16M colors to 16fg/16bg https://github.com/Microsoft/BashOnWindows/issues/76

Note about colour differences: a) RGB axes are not orthogonal, so you cannot use sqrt(R^2+G^2+B^2) formula, b) for colour differences there is more correct (but much more complex) CIEDE2000 formula (which may easily blow up performance if used blindly) [2].

[2] https://github.com/neovim/neovim/issues/793#issuecomment-48106948

Terminal multiplexers

tmux - starting from version 2.2 (support since 427b820...)
screen - has support in 'master' branch, need to be enabled (see 'truecolor' option)
pymux - tmux clone in pure Python (to enable truecolour run pymux with --truecolor option)
dvtm - not yet supporting True Colour https://github.com/martanne/dvtm/issues/10

NOT supporting truecolour

Terminology (Enlightenment) - https://phab.enlightenment.org/T746
mrxvt (looks abandoned) - https://sourceforge.net/p/materm/feature-requests/41/
aterm (looks abandoned) - https://sourceforge.net/p/aterm/feature-requests/23/
fbcon (from linux kernel) - https://bugzilla.kernel.org/show_bug.cgi?id=79551
FreeBSD console - https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=191652
PuTTY (patched version [3] {xterm-like approximation to 256 colors} and [4] {real true colors} available) - Windows platform
libvte and GTK2 - based:
- libvte-based GTKTerm2
- libvte-based stjerm (looks abandoned) - https://github.com/stjerm/stjerm/issues/39

[3] You can download patched version here https://github.com/rdebath/PuTTY

[4] You can download patched version here https://github.com/halcy/PuTTY

Here are another console programs discussions:

Supporting True Colour:

mc - since 682a5.... See also ticket #3742 for truecolor themes.
irssi - since PR #48
neovim - since commit 8dd415e887923f99ab5daaeba9f0303e173dd1aa; need to set termguicolors to enable true color.
vim - (from 7.4.1770) since commit 8a633e3427b47286869aa4b96f2bfc1fe65b25cd; need to set termguicolors to enable true color.
elinks - configure.in:1410 (./configure --enable-true-color)
s-lang library - (since pre2.3.1-35, for 64bit systems)
timg - Terminal Image Viewer
tv - tool to quickly view high-resolution multi-band imagery directly in terminal
mpv - video player with support of console-only output (since https://github.com/mpv-player/mpv/commit/dd02369c3223fda5bcb2658b15404d43232bb38f)

Not supporting True Colour:

mutt - http://dev.mutt.org/trac/ticket/3674
neomutt - https://github.com/neomutt/neomutt/issues/85
ncurses library - https://lists.gnu.org/archive/html/bug-ncurses/2013-10/msg00007.html
termbox library - https://github.com/nsf/termbox/issues/37
mcabber - https://bitbucket.org/McKael/mcabber-crew/issue/126/support-for-true-color-16-millions-colors
emacs - http://emacs.1067599.n5.nabble.com/RFC-Add-tty-True-Color-support-td299962.html and http://emacs.1067599.n5.nabble.com/bug-20243-True-color-terminal-support-tc354040.html