GOLEM/golem_activity_registration_state/views/golem_activity_registration_views.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2016-2018 Fabien Bourgeois <fabien@yaltik.com>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.
-->
<odoo>
  <data>

    <!-- Tree -->
    <record id="golem_activity_registration_tree_inherit_state"
      model="ir.ui.view">
      <field name="name">Session registration list</field>
      <field name="model">golem.activity.registration</field>
      <field name="inherit_id"
        ref="golem_activity_registration.golem_activity_registration_tree" />
      <field name="arch" type="xml">
        <tree position="attributes">
          <attribute name="colors">red: state == 'canceled';darkgrey: state == 'draft';</attribute>
        </tree>
        <tree position="inside">
          <field name="state" invisible="1" />
        </tree>
      </field>
    </record>

  </data>
</odoo>