If in pure water 2 copper electrodes were used to pass a current through it and the electrodes / water did not turn green, but stayed their copper color, what would that mean.

Yes I know pure water is not conductive.

 

You have to do this by Regulating the Current.
A "CL25" can handle up to ~90-Volts with adequate cooling,
( and only if You use the TO-252 (D-PAK) Package ),
and it will limit the Current to 25-mA which will prevent simply "boiling" the Water.

The process may take a long time to get started.

Higher-Voltage will make the process much faster, but You MUST limit the Current.

There may be certain minerals that can be used to speed-up the process.
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If in pure water 2 copper electrodes were used to pass a current through it and the electrodes / water did not turn green, but stayed their copper color, what would that mean.

Yes I know pure water is not conductive.

It would mean there is very little current passing through the water. Not enough to free any oxygen from the water. The green color on copper is copper oxide, and takes oxygen to make it.