You haven't given enough details to answer the question.
the expriment was fill half-a testube with water. then, add 5 drops of bromothymol blue. then, blow it until the color change. initial color was green. the color changed to yellow.now, The color change of the bromothymol blue with water is caused by carbon dioxide from your breath. How can you test this hypothesis?
One thing to try is the following:
Fill a tube with something that will absorb the CO2 in the breath (such as a straw partially filled with ascarite and partially filled with dehydriter (one to absorb CO2 the other other to absorb the water from the reaction--because the water produced in the reaction as well as the moisture in the breath will dissolve CO2). Blow through that straw and see if the indicator changes color. That won't prove conclusively that CO2 is the culprit but it will get you closer to know that.
Be sure the straw is large enough in diameter to hold enough ascarite and dehydrite to do the job; otherwise, you will use all of the reagents after which the straw will begin "leaking" water and CO2.
However, can I do the following: I pour some water in a test tube and apply a pH paper. Then, I blow in the water. If the color changes, then we can be sure that carbon dioxide can change alkalinity or acidity of water. Let me know what you think. I do not have chance to do the procedure described above but in critical thinking, would this be logical?