PDF::Writer：Color::CMYK#pdf_fill, #pdf_stroke

require 'color'

black = Color::CMYK.new(0, 0, 0, 100)
p black.pdf_fill
p black.pdf_stroke

で、

"0.000 0.000 0.000 1.000 k"
"0.000 0.000 0.000 1.000 K"

version 2.0 でなくなるらしい