Make water change Color?

I saw an experiment a few years back where 10 people were selected and given glasses of water. They were to mix their water with 2 other people and at the end there was another chemical added to the cups of water which made the water change color. The purpose was to show the transmission of infectious diease such as HIV. Any idea on how i may be able to replicate this experiment. Thanks!

Phenolphthalein is colorless and turns pink with alkali. Some people can have phenolphthalein solution, and some people can have sodium bicarbonate (baking soda) solution, and some people can have water. When the phenolphthalein solution and soda solution mix, the mixture turns pink.

I'm not sure what chemicals would produce the blue color, but you could to the same experiment with acid and pH paper. One cup of the 10 would need to be "infected" with the "disease" represented by some acid (HCl would work best probably because it would still be strong after mixing with other water glasses). Every other cup should just have water in it and then everyone shares with 2 other people (make sure they mix around the room though) then when everyone has finished, get out the pH paper and test to see how many cups are acidic or "infected."

Oh, I just rememberd, bromothymol blue, BTB is an indicator that will change a solution with a weak acid yellow or green. You would just need to do the same experiment with the acid and then drop in a drop or two of BTB to see the color change.