As turmeric powder is present in vegetable and when it comes in contact with caustic soda in soap.

It results in chemical reaction which turns red in color because caustic soda is alkaline in nature.

Vegetable stain turns reddish-brown when washed with soap because vegetable stain contains turmeric. 

When soap solution is added to it, turmeric turns into reddish-brown color because the soap solution is basic in nature, and on washing off the soap with water the medium is neutral again.

Turmeric is a natural indicator which changes its yellow colour into reddish brown when it comes in contact with a solution of base. Hence, when soap (which is basic in nature) is scrubbed on the stain, the stain turns red.