PDF::Writer：Color::RGB#adjust_brightness

require 'color'

p Color::RGB::Red.adjust_brightness(10)
p Color::RGB::Green.adjust_brightness(10)
p Color::RGB::Blue.adjust_brightness(10)

p Color::RGB::Red.adjust_brightness(-10)
p Color::RGB::Green.adjust_brightness(-10)
p Color::RGB::Blue.adjust_brightness(-10)

で、

#<Color::RGB:0x40228a90 @r=1.0, @b=0.0999999999999995, @g=0.1>
#<Color::RGB:0x4022839c @r=0.0, @b=0.552156862745098, @g=0.00862745098039203>
#<Color::RGB:0x40227cbc @r=0.1, @b=1.0, @g=0.1>
#<Color::RGB:0x40227618 @r=0.9, @b=0.0, @g=0.0>
#<Color::RGB:0x40226f38 @r=0.0, @b=0.451764705882353, @g=0.00705882352941167>
#<Color::RGB:0x4022686c @r=0.0, @b=0.9, @g=0.0>