An indicator is a weak acid that has a different colour when it is the acid or the salt.

HIn  ⇌   H+ + In

The colour change occurs when either the salt or the weak acid are in majority, and therefore for a distinct colour change, a sharp drop or gain in the pH level is necessary.

When pka = pH the salt and the acid would have the same concentration, which gives a rough indication of when the colour change will take place. This can be derived as follows:

Ka = [H+][In]                                                                              [HIn]

The colour change would take place when [HIn] = [In] which would leave pka = pH