My tub got stained?
This can happen especially with older tubs that are a bit porous.
To resolve this, if you fill the tub (above the line where you're seeing colour) and add about 1/2 cup of baking soda and let it soak, it will help the colour to lift. After that, take a lemon, cut it in half, and rub it over the affected area.
This should help remove any colour left over!