From: HowtoConfigureLogging on the Rails Wiki site.

Getting monochrome log messages

Removing the colour escape sequences that are used by default is quite
simple. Simply set colorize_logging to false:

ActiveRecord::Base.colorize_logging = false

Note that the colorize_logging method doesn't appear in
ActionController (why?[because the only module in rails that adds
color to the logs is ActiveRecord -jean]).