Yellow spice is one of four spices used on stew to make spicy stew. The Freeing Evil Dave subquest of Recipe for Disaster must be completed to obtain these in Evil Dave's Basement of Doom in Edgeville. Sending your cat to catch Hell-Rats yields between 1 to 4 doses of any of the spice colours and the Hell-Rat Behemoth in the south-western room always drop 4 doses of the spice they guard. Spices of the same colour can be used on each other to combine up to 4 doses to save space. Once all doses are used it becomes an empty spice shaker.
Spicy stews containing yellow spice temporarily boost or drain the stats of Agility, Hunter, Slayer, Thieving, Prayer, and Sailing. A single dose causes those stats to increase or decrease by a maximum of +1 or -1, while two doses affect those stats by -3 or +3, up to a maximum of -5 or +5 with three doses. Yellow spice is mainly useful for boosting Thieving or Sailing. Boosting Prayer only increases usable Prayer points, whereas Agility and Slayer have access to the summer pie and wild pie, respectively, which grant a consistent +5 boost per bite. Similarly, Hunter has access to the Super hunter potion, which grants a consistent +6 boost per sip.
The locations of the coloured spices the Behemoths guard.