color-log gem: Colorizes the output of the standard library debug logger. http://blog.champierre.com/1064

color-log.gemspec

Gem::Specification.new do |spec|
 spec.name = "color-log"
 spec.version = "0.0.1"
 spec.authors = ["Junya Ishihara"]
 spec.email = ["webmaster@champierre.com"]
 spec.summary = "Colorizes the output of the standard library debug logger."
 spec.files = ["color-log.rb"]
 spec.require_path = "."
end

color-log.rb

module ActiveSupport
  class Logger < ::Logger
    class SimpleFormatter < ::Logger::Formatter
      def call_with_debug_colored(severity, timestamp, progname, msg)
        if Rails.env.development? && severity == 'DEBUG'
          "\033[33m#{call_without_debug_colored(severity, timestamp, progname, msg)}\033[0m"
        else
          call_without_debug_colored(severity, timestamp, progname, msg)
        end
      end
      alias_method_chain :call, :debug_colored
    end
  end
end