It's very simple tool (written in python), that basically colorizing (and do some other formatting too) any output that you can get: tail, less, vi, ping, ... .
Plain boring tail output:
Same output with rainbow parameter
--red='.*ERROR.*', which means color all text red that matches regular expression
One more example:
--blue='irina'(make all mentions about me blue):
With rainbow you can also make config files, where you can describe how you want to process some file. Here is a simple configuration file:
I've tried many different tools for colorizing logs and rainbow seems the best for me: it's very simple (you don't need to learn new syntax), it can work both individually and with configuration files, it works with any output that I need to make.