[gnome-software.wiki] Update Software metadata

[Philip Withnall <pwithnall src gnome org>]

commit a25d177e9b97481c4e572ee2a332450deb80209b
Author: Philip Withnall <philip tecnocode co uk>
Date:   Thu Sep 9 14:52:42 2021 +0000

    Update Software metadata

 Software-metadata.md | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)
---
diff --git a/Software-metadata.md b/Software-metadata.md
index 402c6fc..88d4417 100644
--- a/Software-metadata.md
+++ b/Software-metadata.md
@@ -156,5 +156,20 @@ How to fix incorrect safety and permissions information
 How to set the carousel tile background colour
 ---
 
- - 
https://gitlab.gnome.org/GNOME/gnome-software/-/blob/eac007a8bf87207347ab2c8baf629d5d58aa0c6b/lib/gs-app.c#L4239-4248
- - TODO
\ No newline at end of file
+The carousel tile background colour is normally extracted automatically from the application’s icon. If that 
colour is unsatisfactory, however, it can be overridden by adding the following XML to the application’s 
metainfo `<component>` element:
+
+```xml
+<custom>
+  <value key="GnomeSoftware::key-colors">[(124, 53, 77), (99, 16, 0)]</value>
+</custom>
+```
+
+The value of `GnomeSoftware::key-colors` is a text-format GVariant with type `a(yyy)`, and represents an 
unordered set of key colours for the application. Each element of the set is a colour in RGB form ranging 
from `(0, 0, 0)` to `(255, 255, 255)`.
+
+Colours should be chosen so that they:
+ * Are identifiable to the application, matching the application’s branding in some way, if possible.
+ * Contrast with the edge of the application’s icon, so that when they are used as a background behind it, 
the icon can be distinguished.
+ * 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.
\ No newline at end of file