electrochemical cell question help please

(3) (c) You are provided with the Daniell cell referred to in part (b), including a zinc electrode of
known mass.
Briefly outline how you would carry out an experiment to confirm your answer to part (b).

Mark scheme:
Allow cell to discharge until [Cu2+] is 0.5 mol dm-3
Confirmed by colorimetric measurement
Weigh the Zn electrode before and after the experiment.

What does discharge exactly mean? And how would colorimetry confirm that the Cu2+ concentration had fallen to 0.5?

(Question b says: A Daniell cell was set up using 100 cm3 of a 1.0 mol dm– 3 copper(II) sulfate solution. The cell was allowed to produce electricity until the concentration of the copper(II) ions had decreased to
0.50moldm–3.
Calculate the decrease in mass of the zinc electrode. (The answer is 3.27g))

discharge in this context just means react.

Colorimetry is done by comparing the sample to reference solutions, or to a calibration curve produced by reference solutions

so is there a colour to measure in this case?

Has to go down as a pretty daft question. Colourimetry

well yeah I obviously know that man its just I'm confused what colour is being measured and of what?