We should know that the color of many substances changes as the pH changes, this is because the chemical structure of the substance is affected by the pH. This is why we see the color change in the image on the right.
In the case of the buffered solution, when small amounts of acid or base are added, the change in pH is not significant. This is because a buffer is a chemical system that affects the concentration of hydrogen ions (or hydroniums) in a solution and the pH is directly related to the concentration of hydrogen ions in the solution.
So, as in the buffered solution, the pH remains constant, the color also remains the same as seen in the image on the left.