What is Acid–base indicator, colour-change range?

An acid–base indicator is a couple $\ce{HInd}$/$\ce{Ind-}$ whose two forms have different colours, used in very small amounts. Its colour-change range is the pH interval over which the colour is seen to change, roughly $pK_a - 1$ to $pK_a + 1$: below, the colour of $\ce{HInd}$ is seen, above, that of $\ce{Ind-}$, and in between a mixture of the two.