How to change color of water based on its acidity? I recently saw a video on YouTube in which water, when mixed with a solution (I believe iodine was involved,) changed color based on its acidity. I am attempting to replicate this experiment, but cannot concoct the solution. Any help would be much appreciated.

EDIT:
To clarify, if I had water with a ph of 10 it would turn red, but if the water had a ph of 4 it would turn green.

EDIT 2:
I found the video in question 
http://youtu.be/_yi83kOB8ug
I'm assuming that you're doing this at home. A popular (and SAFE!!) experiment at home is to use red cabbage, vinegar, and baking soda. 

The indicator comes from the cabbage.

Vinegar turns the solution acidic.

Baking soda turns the solution more alkaline.