GOLEM/golem_activity_place/views/golem_activity_view.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2016 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>

    <!-- Form -->
    <record id="view_form" model="ir.ui.view">
      <field name="name">GOLEM Activity Places Form additions</field>
      <field name="model">golem.activity</field>
      <field name="inherit_id"
        ref="golem_activity_registration.activity_form" />
      <field name="arch" type="xml">
        <field name="places" position="after">
          <field name="places_min" />
          <field name="is_overbooked" />
          <field name="places_overbooked"
            attrs="{'invisible': [('is_overbooked', '=', False)]}" />
        </field>
      </field>
    </record>

    <!-- Tree -->
    <record id="tree" model="ir.ui.view">
      <field name="name">Activity list places adaptations</field>
      <field name="model">golem.activity</field>
      <field name="inherit_id"
        ref="golem_activity_registration.activity_tree" />
      <field name="arch" type="xml">
        <tree position="attributes">
          <attribute name="colors">darkgrey: places_used &lt; places_min; red: places_remain == 0; orange: places_remain &lt;= 4;</attribute>
        </tree>
        <field name="places_remain" position="after">
          <field name="places_used" invisible="True" />
          <field name="places_min" invisible="True" />
        </field>
      </field>
    </record>

  </data>
</odoo>