title: Random document from a specific field query
body: <p>I have a collection with a document structure as this:</p>
  
  <pre><code>{_id:"XXXXXXXXX", contents:{...}, color:"red"}
  </code></pre>
  
  <p>I would like to get a <strong>random document with a specific color</strong>. For instance, a random document that's also "red".</p>
  
  <p>How can I achieve this?</p>