Add coloring script output details to GitLab CI YAML docs

doc/ci/yaml/README.md

@@ -661,6 +661,44 @@ job:
 
 [YAML anchors for `before_script` and `after_script`](#yaml-anchors-for-before_script-and-after_script) are available.
 
+#### Coloring script output
+
+Script output can be colored using [ANSI escape codes](https://en.wikipedia.org/wiki/ANSI_escape_code#Colors),
+or by running commands or programs that output ANSI escape codes.
+
+For example, using [Bash with color codes](https://misc.flogisoft.com/bash/tip_colors_and_formatting):
+
+```yaml
+job:
+  script:
+    - echo -e "\e[31mThis text is red,\e[0m but this text isn't\e[31m however this text is red again."
+```
+
+You can define the color codes in Shell variables, or even [custom environment variables](../variables/README.md#custom-environment-variables),
+which makes the commands easier to read and reusable.
+
+For example, using the same example as above and variables defined in a `before_script`:
+
+```yaml
+job:
+  before_script:
+    - TXT_RED="\e[31m" && TXT_CLEAR="\e[0m"
+  script:
+    - echo -e "${TXT_RED}This text is red,${TXT_CLEAR} but this part isn't${TXT_RED} however this part is again."
+    - echo "This text is not colored"
+```
+
+Or with [PowerShell color codes](https://superuser.com/a/1259916):
+
+```yaml
+job:
+  before_script:
+    - $esc="$([char]27)"; $TXT_RED="$esc[31m"; $TXT_CLEAR="$esc[0m"
+  script:
+    - Write-Host $TXT_RED"This text is red,"$TXT_CLEAR" but this text isn't"$TXT_RED" however this text is red again."
+    - Write-Host "This text is not colored"
+```
+
 ### `stage`
 
 `stage` is defined per-job and relies on [`stages`](#stages) which is defined