Phenol red is a ph indicator routinely used to monitor the ph of solutions in laboratories. It is reddish-orange in neutral solutions, yellow in acidic solutions, and magenta in basic solutions. What color would a neutral, phenol-red containing solution turn if you started blowing your exhaled air into it using a straw?

The answer is yellow.

That is if you started blowing your exhaled air into a phenol-red containing solution, then it color will change to yellow.

It changes it color as we blow exhaled air into it as we are introducing carbon dioxide into the mixture.

And as carbon dioxide concentration increases, the H ion concentration increases that means the solution becomes acidic that is it becomes yellow.