In my dictionary it defines
黃 (Huáng) - yellow - pornographic - to fall through - surname Huang or Hwang
Imperial China and the robes of the emperor are commonly associated with the colour yellow.
We also have
黃河 - Huánghé - the yellow river 黃山 - Huángshān - the yellow mountain 黃帝 - Huángdì - the legendary Yellow Emperor
So how did the colour yellow come to be associated with pornography?
黃色電影 - Huángsè diànyǐng - pornographic movie 黃色書刊 - Huángsè shūkān - pornographic book