[gnome-software.wiki] Add tips for testing carousel tile background colour

[Will Thompson <wjt src gnome org>]

commit b45225e7300a3088880a544a09acc4fe3c1b4d98
Author: Will Thompson <will willthompson co uk>
Date:   Fri Jan 21 10:27:59 2022 +0000

    Add tips for testing carousel tile background colour

 Software-metadata.md | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)
---
diff --git a/Software-metadata.md b/Software-metadata.md
index d7f8ed3..c7e24aa 100644
--- a/Software-metadata.md
+++ b/Software-metadata.md
@@ -225,4 +225,31 @@ Colours should be chosen so that they:
  * Contrast with the foreground text colour in the Adwaita theme (`#2e3436`) and in its dark variant 
(`#eeeeec`). This generally means choosing a colour with a medium level of brightness.
  * Contrast can be checked using the [Contrast app](https://apps.gnome.org/app/org.gnome.design.Contrast/).
 
-At least one colour should be specified. More can be specified if one colour can’t satisfy all the 
requirements above, and GNOME Software will choose the most appropriate to use in the carousel tile.
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+At least one colour should be specified. More can be specified if one colour can’t satisfy all the 
requirements above, and GNOME Software will choose the most appropriate to use in the carousel tile.
+
+### How to test carousel tile background colour changes
+
+On some distributions, you can test your changes by editing 
`/usr/share/app-info/xmls/org.gnome.Software.Featured.xml`. (On GNOME OS, run `sudo ostree admin unlock` to 
be able to temporarily edit this file.) Remove all the other apps, and add a section for your app, as follows
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<components>
+  <component merge="append">
+    <id>org.freedesktop.Bustle</id>
+    <categories>
+      <category>Featured</category>
+    </categories>
+    <custom>
+      <value key="GnomeSoftware::FeatureTile">True</value>
+      <value key="GnomeSoftware::key-colors">[(20, 200, 235)]</value>
+    </custom>
+  </component>
+</components>
+```
+
+Now restart gnome-software:
+
+```
+gnome-software --quit
+gnome-software
+```
\ No newline at end of file