yellow colour

erotic, pornographic, obscene, pertains to events or items associated with sex (orig. borrowed from “yellow journalism”, now used more widely) [粵]: 鹹濕

a vice den

(noun) a personal hangout; someone's home; den; hideaway, usually referring to an illegal underground den used for activities such as gambling or prostitution(e.g. "黄色架步" or "識情架步"= a prostitution parlour); means / artifice; unspoken rules; the procedures; the line-up (its extent / scale represents the degree of importance placed on it)