Indicators are weak acids caught in their own equilibrium. The molecule itself shifts between two forms with wildly different colours, and the ratio between them is set entirely by the pH around it. Raise the pH and watch phenolphthalein transform as pH crosses its transition range.
Colour ratio = acid form : base form (%)
Phenolphthalein changes colour between about pH 8.3 and 10.0. Its pKa, the pH where half the molecules are in each form, is about 9.3. Below pH 8.3 more than nine in ten molecules are in the colourless acid form, so the solution looks colourless. As pH rises past 8.3 the equilibrium shifts and the pink base form shows. By pH 10 more than four in five molecules have lost their proton and the solution is deep pink. (In very strong base, above about pH 12, the pink slowly fades again; the picture leaves that out.) This makes it a good choice for strong acid–strong base titrations, where the equivalence point is pH 7 and the pH jumps steeply through its range, and the standard choice for weak acid–strong base titrations, where the equivalence point sits around pH 8–9. It is a poor choice for strong acid–weak base titrations, where the equivalence point is near pH 5, below its range. Where it changes is set by the indicator itself, not by the strength of the acids you are titrating.
An indicator changes colour only across its narrow transition range. Choose one whose range sits inside the steep pH jump at the titration's equivalence point, or the colour will change at the wrong volume.