web/web_editor_background_color/views/snippets.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- Copyright 2017 Jairo Llopis <jairo.llopis@tecnativa.com>
     License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl.html). -->

<odoo>

    <template id="snippet_options"
              inherit_id="web_editor.snippet_options">

        <xpath expr=".">
            <!-- The same option that already exists in website editor, but
                 enabled for mass mailing editor -->
            <div data-js='colorpicker'
                 data-selector=".bg-color, #editable_area > div, a.o_default_snippet_text">
                <li class="dropdown-submenu">
                    <a tabindex="-1" href="#">Color</a>
                    <ul class="dropdown-menu">
                        <li></li>
                    </ul>
                </li>
            </div>
        </xpath>
    </template>

</odoo>