A student carries out a titration to determine the concentration of a sodium hydroxide ($NaOH$) solution. Using a volumetric pipette, 25.0 cm³ of 0.150 mol dm^-3 nitric acid (${HNO}_3$) is transferred to a conical flask. Phenolphthalein indicator is added to the flask. The $NaOH$ solution is placed in a burette and titrated into the flask.

Which of the following correctly describes the colour change at the end point and the effect on the calculated concentration of $NaOH$ if the student had rinsed the conical flask with the sodium hydroxide solution instead of distilled water before adding the acid?