Thymol blue is a pH indicator that changes colour from red to yellow in the pH range 1.2-2.8. Phenolphthalein is a pH indicator that changes colour from colourless to pink in the pH range 8.2-10.0.

If you add hydrochloric acid to a mixture of thymol blue and phenolphthalein, the solution will turn red due to the thymol blue indicator changing colour. The phenolphthalein indicator will not change colour as the pH of the solution will be below 8.2.