The allusion was to certain practices having to do with the merchandising of Red Silk Girls, in private sales for individual and important clients of the House.

She was attired in a brief gown of red Pleasure Silk, prescribed for her because she was a Red Silk Girl and in training.
Assassin of Gor Book 5 Page 176

Later, in the eighteenth week of their training, they were given brief silken slave livery, sleeveless, fastened by the loop on the left shoulder. Virginia and Phyllis were given white livery, Elizabeth red.
Assassin of Gor Book 5 Page 199 200

"She is only white silk," said Lana. "Lana is red silk. Let Lana please you."
Captive of Gor Book 7 Page 114

"White-silk, She-sleen," he said. "Wait until you are red silk!"
Captive of Gor Book 7 Page 186