As String class methods (unix only):
class String
def black; "\e[30m#{self}\e[0m" end
def red; "\e[31m#{self}\e[0m" end
def green; "\e[32m#{self}\e[0m" end
def brown; "\e[33m#{self}\e[0m" end
def blue; "\e[34m#{self}\e[0m" end
def magenta; "\e[35m#{self}\e[0m" end
def cyan; "\e[36m#{self}\e[0m" end
def gray; "\e[37m#{self}\e[0m" end
def bg_black; "\e[40m#{self}\e[0m" end
def bg_red; "\e[41m#{self}\e[0m" end
def bg_green; "\e[42m#{self}\e[0m" end
def bg_brown; "\e[43m#{self}\e[0m" end
def bg_blue; "\e[44m#{self}\e[0m" end
def bg_magenta; "\e[45m#{self}\e[0m" end
def bg_cyan; "\e[46m#{self}\e[0m" end
def bg_gray; "\e[47m#{self}\e[0m" end
def bold; "\e[1m#{self}\e[22m" end
def italic; "\e[3m#{self}\e[23m" end
def underline; "\e[4m#{self}\e[24m" end
def blink; "\e[5m#{self}\e[25m" end
def reverse_color; "\e[7m#{self}\e[27m" end
end
and usage:
puts "I'm back green".bg_green
puts "I'm red and back cyan".red.bg_cyan
puts "I'm bold and green and backround red".bold.green.bg_red
on my console:

additional:
def no_colors
self.gsub /\e\[\d+m/, ""
end
removes formatting characters
Note
puts "\e[31m" # set format (red foreground)
puts "\e[0m" # clear format
puts "green-#{"red".red}-green".green # will be green-red-normal, because of \e[0