Page "PH indicator" Paragraph 8

For optimal accuracy, the color difference between the two species should be as clear as possible, and the narrower the pH range of the color change the better.

In some indicators, such as phenolphthalein, one of the species is colorless, whereas in other indicators, such as methyl red, both species confer a color.

While pH indicators work efficiently at their designated pH range, they are usually destroyed at the extreme ends of the pH scale due to undesired side-reactions.