Question

why does a vegetable stain turn reddish brown when washed with soap solution

Answer 1

because soap contain bases and vegetables are indicators of acids and bases

Answer 2

Hello!
Turmeric is a PH indicator which displays different colour in acidic basic and neutral solutions It is Yellow Colour in Acidic and Neutral medium and in basic It turns Red brown.
Soap is basic and in presence of soap, Turmeric is reddish brown.
Hope its Help u