How do you know what colour changes have occurred in a galvanic cell?

For example,

A Nickel metal placed in a solution of Ni(NO3)2 connected to a platinum metal placed in a solution of KCl, where chlorine gas is bubbled through.

The nickel nitrate solution is pale green and the KCl is colourlessHow do you know what colour changes have occurred in a galvanic cell?
The only thing that will affect the colour is the nickel nitrate solution getting weaker [less green].

So once you can decide what happens to the nickel nitrate in your galvanic cell, you have the answer

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