phpunit xml configuration colors instruction dosen't work

I define a phpunit.xml configuration file for my unit tests. In this file I instruct to show colors in the unit tests relatory like this:

<phpunit
   <!-- ... -->
   colors="true"
  <!-- ... -->
</phpunit>

When I run the tests in the command line Is all ok except that I get no green and red colors in the results. If I delete the configuration file and use the parameter --color=always in the command line then I get the result with colors. Whats the problem in the configuration file?