Experiment with different quantities for the effect you want. The first solution should be plain water. Adding a few drops of iodine will turn it golden yellow. The next solution should be the washing soda with a little starch water mixed in. If you use the starch and iodine sparingly you get a nice navy blue for U.S. patriotic themes.
Larger concentrations look nearly black.
Experiment to get just the effect you want. The next solution should be hypo with a little Ex-Lax solution mixed in. The hypo combines with the iodine to cancel out the starch reaction and make the solution clear. The final solution is the white vinegar, which neutralizes the ammonia and makes everything clear again. Presentation A color changing liquid is only a chemical curiosity unless there is a story to support it.