web/web_editor_background_color/templates/editor.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2019 Tecnativa - Jairo Llopis
     License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl). -->
<data>
    <template id="colorpicker" inherit_id="web_editor.colorpicker">
        <xpath expr="//colorpicker" position="inside">
            <div class="o_colorpicker_section" data-name="custom" data-display="Custom Color"/>
        </xpath>
    </template>
</data>