Highlighting with grep doesn't work in xterm

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http://askubuntu.com – Hi askubunutu community, I've got a problem with xterm. When I use the command tail -F example.log | grep -a -i -e 'examplestring' in xterm the "grepped" strings aren't highlighted in red like they would be in the normal terminal. How can i fix this? Google didn't help me very much. (Sorry for my bad english) Greetings, Bannix (HowTos)