Colours
From wipipedia.org

Various colours have significance in BDSM and fetish, for example:
Red
1. A commonly used safeword.
2. A colour used in the Gay Hanky Code to indicate an interest in fisting.
3. Red silk (Gorean term)
Yellow
1. A commonly used safeword to indicate a need to "slow down" the intensity of an activity, reflecting the red, yellow (or amber), green of traffic lights.
2. The colour used in the Gay Hanky Code to indicate interest in watersports.