Indicators can be considered a reference to an equilibrium in solution, which is influenced by the concentration of hydronium ions. ($H_{3}O^{+}$)
General reaction is as follows:
$$\begin{align} (1) \ \ In^{-}+H_{3}O^{+}\rightleftharpoons HIn + H_{2}O \end{align} $$It can also be written the other way:
$$\begin{align} HIn + H_{2}O \rightleftharpoons In^{-}+H_{3}O^{+} \end{align} $$Here, $In^{-}$ is the conjugate base of the indicator, while $HIn$ is the conjugate acid of the indicator.
For an indicator to be useful, $In^{-} \text{ and } HIn$ must have different colours (for example, for litmus $HIn$ is red, $In^{-}$ is blue).
This is what happens when we increase $[H_{3}O^{+}]$
The inverse is true if we were to decrease $[H_{3}O^{+}]$, say if $OH^{-}$ was added.
UI is good for a general, broad idea of the pH. But for most indicators, there's only 1 change in colour(i.e. from blue to red)
