PDF::Writer：Color::CMYK.new

require 'color'

black = Color::CMYK.new(0, 0, 0, 100)
p black
p (black == Color::RGB::Black)

で、

#<Color::CMYK:0x401cba70 @y=0.0, @m=0.0, @c=0.0, @k=1.0>
true