title: How to color a field in tree view with today as filter
body: <p>I am  new to openERP.  I am working on changing the color in tree view for those record whose due_date is less then Today.  Those record should be in red color.  </p>
  
  <p>I am not able to get the current date as filter in tree view.</p>
  
  <p>xml what I need but <strong>not working</strong>: </p>
  
  <pre><code>&lt;tree colors="blue:Due_date &lt; Today()" string="Invoice"&gt;
                      &lt;field name="Due_date" /&gt;
                      &lt;field name="date_invoice"/&gt;
                      &lt;field name="state"/&gt;
                  &lt;/tree&gt;
  </code></pre>
  
  <p><strong>Working code</strong></p>
  
  <pre><code>&lt;tree colors="blue:state == 'open'" string="Invoice"&gt;
                      &lt;field name="partner_id" groups="base.group_user"/&gt;
                      &lt;field name="date_invoice"/&gt;                      
                      &lt;field name="state"/&gt;
                  &lt;/tree&gt;
  </code></pre>
  
  <p>I tried with </p>
  
  <pre><code>context_today()
  </code></pre>
  
  <p>and</p>
  
  <pre><code>datetime.datetime.now()
  </code></pre>
  
  <p>Nothing work.</p>
  
  <p>Please help me on this.</p>