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