Why does sea salt turn brown?

Iodine is used to find starch. If no starch is present, a rusty brown color will appear. If starch is present like in bread and potato, the iodine when put on will turn blackish blue. That is known as a chemical reaction.

Since there is no starch in water, the iodine in the water stays the same color as it started (brown).

Hope that helps!! :)