Turmeric is a type of powder which belongs to ginger family and is called zingiberaceae. Turmeric is used both as spice and as a natural Dyce also. The yellow colour of turmeric is due to presence of compound cur cumin.
This compound which is present in turmeric is due to hydroxyl groups. This compound does not undergo any change in an acidic medium but it changes in basic medium.
In addition to this compound, there is another compound called phenolphthalein which resembles this substance. This phenolphthalein is colourless in nature. Both this substance does not change its colour in acidic medium but in basic medium turmeric turns reddish brown and phenolphthalein turns pink.
Another substance which does not changes its colour in basic medium but changes its colour in acidic medium in methyl orange. It turns pink in acidic medium. Thus, turmeric powder turns red because slaked lime which is nothing but calcium hydroxide is added to it and it becomes red. When any such substance will be added to red turmeric will turn yellow. This is the reason why turmeric powder turns red when mixed with slaked lime.