From 'emacs -Q', this is what I did:

- (add-hook 'compilation-filter-hook #'ansi-color-compilation-filter)
- M-x rgrep RET
- setq RET
- RET
- ~/emacs RET (this is where I keep the entire Emacs source code)

As you can see from the attached image, some of the results are red and some are 
not. This is not happening without ansi-color-compilation-filter.y

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.18, cairo 
version 1.16.0)
  of 2021-08-26 built on hathaway
Repository revision: 4ac29b943bdcc099f578660395b17b430551ff79
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Ubuntu 20.04 LTS

Configured using:
  'configure --with-harfbuzz --with-native-compilation'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
   value of $LC_MESSAGES: en_GB.UTF-8
   value of $LC_MONETARY: it_IT.UTF-8
   value of $LC_NUMERIC: it_IT.UTF-8
   value of $LC_TIME: it_IT.UTF-8
   value of $LANG: en_US.UTF-8
   value of $XMODIFIERS: @im=ibus
   locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
   tooltip-mode: t
   global-eldoc-mode: t
   eldoc-mode: t
   electric-indent-mode: t
   mouse-wheel-mode: t
   tool-bar-mode: t
   menu-bar-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   blink-cursor-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   line-number-mode: t
   indent-tabs-mode: t
   transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny rfc822 mml mml-sec epa
derived epg epg-config gnus-util rmail rmail-loaddefs mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
time-date find-dired dired dired-loaddefs ffap url-parse auth-source
cl-seq eieio eieio-core cl-macs eieio-loaddefs password-cache json
subr-x map seq byte-opt gv bytecomp byte-compile cconv url-vars
thingatpt cl-loaddefs cl-lib files-x grep compile text-property-search
comint ansi-color ring iso-transl tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process
native-compile emacs)

Memory information:
((conses 16 5121823 158759)
  (symbols 48 7949 0)
  (strings 32 26036 7196)
  (string-bytes 1 889591)
  (vectors 16 79368)
  (vector-slots 8 680334 499758)
  (floats 8 28 57)
  (intervals 56 424949 1659)
  (buffers 992 12))

-- 
Manuel Uberti
www.manueluberti.eu