Not something I've tried before so gave it a whirl.
Added the code below to a new template:
Code:
<customUI
xmlns="http://schemas.microsoft.com/office/2009/07/customui"
xmlns:mso="http://schemas.microsoft.com/office/2009/07/customui" >
<ribbon >
<tabs >
<tab id="newTab" >
<group id="newGroup" >
<mso:gallery idQ="mso:TextHighlightColorPicker" showInRibbon="false" visible="true"/>
</group >
</tab >
</tabs >
</ribbon >
</customUI >
Although this results in an error on load the gallery gets added anyway.
Screenshot 2025-10-29 122254.png
EDIT: It is the parameter "showInRibbon="false" " that creates the error
The top half of the button applies the current highlight color, the bottom half displays the standard gallery.
Screenshot 2025-10-29 122341.png
Double clicking also seems to work, but only after displaying the gallery to set the highlight color.
Hope that helps.
EDIT 2: Attached template contains my working example. Although a dotm there is no code.