A stain of curry on a white cloth become reddish brown when soap is used because the soap is:

Curry contains turmeric, which contains a natural indicator pigment called curcumin. Turmeric remains yellow in neutral or acidic mediums but changes its color to reddish-brown when it comes into contact with a basic (alkaline) substance. Since commercial washing soaps are basic in nature, treating the curry stain with soap turns it reddish-brown.