No such field found: field groovy.util.slurpersupport.NodeChild primary

script {
    def xml = """
              <colors>
                  <color primary="true">Red</color>
                  <color primary="true">Yellow</color>
                  <color primary="true">Blue</color>
                  <color primary="false">Purple</color>
              </colors>
              """

    def colors = new XmlSlurper().parseText(xml)
    echo "First Color: ${colors.color[0]}" //works fine
    echo "First Color: ${colors.color[0]} Primary? ${colors.color[0].@primary}" //fails

}