I find grep\'s --color=always flag to be tremendously useful. However, grep only prints lines with matches (unless you ask for context lines). Give
grep
--color=always
As grep -E '|pattern' has already been suggested, just wanted to clarify it's possible to highlight the whole line too.
grep -E '|pattern'
For example tail -f /somelog | grep --color -E '| \[2].*':
tail -f /somelog | grep --color -E '| \[2].*'