SGR colon sub-parameters — ITU-T T.416 alternative (38:2:: / 48:2:: / 38:5:)

Byte forms

Every common string-literal form so you can paste-and-search either direction.

Description

ITU-T Rec. T.416 / ISO/IEC 8613-6 (1993) — the colon-separator form for SGR sub-parameters. Where xterm's original extended-color introduction used semicolons throughout (\x1b[38;2;R;G;Bm), the formal standard reserves : (0x3a) as the sub-parameter separator within a single SGR parameter, and keeps ; (0x3b) as the parameter separator between independent SGR codes. The shapes are:

- 24-bit truecolor: \x1b[38:2::R:G:Bm (fg) / \x1b[48:2::R:G:Bm (bg). The empty slot after 2 (yielding the ::) is the optional colorspace ID — empty means RGB, but T.416 allows 38:2:Pcs:R:G:B where Pcs identifies CIE Lab / CIE u'v'L' / etc. Most emulators ignore Pcs and treat the slot as decorative. - 256-color: \x1b[38:5:Nm / \x1b[48:5:Nm. Note: only ONE colon between 5 and N here — no empty :: slot, because the 256-color form has no colorspace argument. - CMY (38:3:Pcs:C:M:Y) and CMYK (38:4:Pcs:C:M:Y:K) variants exist in T.416 but are essentially never implemented.

Why this matters: the original xterm form \x1b[38;2;R;G;Bm is ambiguous to a strictly-conforming SGR parser. Each semicolon-separated token is supposed to be an independent SGR code, so a strict parser sees 38;2;R;G;B as five independent codes: 38 (set fg to *next item*), 2 (which it might interpret as 'faint'!), then R G B as more SGR codes. The colon form 38:2::R:G:B is unambiguous because the entire compound is a single parameter containing sub-tokens — the colons clearly nest under the 38. The conformance breakdown:

- Honour the colon form: kitty (since 0.17), foot, WezTerm, alacritty (since 0.10), ghostty, iTerm2, mlterm, konsole 22.04+, xterm 273+, gnome-terminal recent. - Honour only the semicolon form: very old emulators, Windows ConPTY (older), some embedded VT clones. - Honour both: every modern emulator in the first list — they parse ; and : as equivalent in the truecolor / 256-color slot to avoid breaking existing software.

T.416 also defines colon sub-parameters for SGR 4 underline styles — \x1b[4:0m (none), \x1b[4:1m (single, same as 4m), \x1b[4:2m (double), \x1b[4:3m (curly / wavy), \x1b[4:4m (dotted), \x1b[4:5m (dashed) — and the underline colour pair \x1b[58:2::R:G:Bm (set underline colour to RGB) / \x1b[59m (default underline colour). The wavy underline is the killer feature here — IDE-style 'misspelled word' visuals at the terminal layer.

For portable software, the safe pattern is to emit the semicolon form (still understood by every emulator) but parse both forms when consuming ANSI bytes from an arbitrary log file.

Spec citation: ITU-T Rec. T.416 (ISO/IEC 8613-6) / xterm-ctlseqs (SGR Pm)

Parameters

Examples

# Strict T.416 truecolor (note the `::`):\nprintf '\033[38:2::255:128:64morange text\033[0m\n'\n# Curly underline for spell-check overlays:\nprintf '\033[4:3mtypo\033[0m\n'

import sys\nRGB = (255, 128, 64)\nsys.stdout.write(f'\x1b[38:2::{RGB[0]}:{RGB[1]}:{RGB[2]}morange\x1b[0m\n')\n# Underline colour (T.416 58 / 59):\nsys.stdout.write('\x1b[4m\x1b[58:2::255:0:0merror\x1b[59m\x1b[24m\n')

// Emit semicolon form (most portable) but parse both:\nfmt.Print("\x1b[38;2;255;128;64morange\x1b[0m\n")\n// T.416 strict form:\nfmt.Print("\x1b[38:2::255:128:64morange\x1b[0m\n")

// Curly underline + RGB underline colour:\nprocess.stdout.write('\x1b[4:3m\x1b[58:2::200:80:80mmisspelled\x1b[59m\x1b[24m\n')

/* T.416 256-color: single colon, no empty slot: */\nprintf("\x1b[38:5:208morange-216\x1b[0m\n");\n/* Truecolor with empty colorspace slot: */\nprintf("\x1b[48:2::20:20:60mdeep blue bg\x1b[0m\n");

Terminal support

xterm

yes

Linux console (fbcon)

no

macOS Terminal.app

no

iTerm2

yes

Windows Terminal

partial

cmd.exe / ConPTY

no

kitty

yes

alacritty

yes

WezTerm

yes

Ghostty

yes

GNOME Terminal

yes

Konsole

yes

tmux

partial

GNU screen

no

Related sequences

• SGR 38;2;R;G;B — 24-bit truecolor foreground
• SGR 48;2;R;G;B — 24-bit truecolor background
• SGR 38;5;n — 256-color foreground
• SGR 48;5;n — 256-color background
• SGR 4 — Underline

In the family cookbook

↗SGR cookbook · 6. SGR colon sub-params — the standards-correct form