You are correct. As the oxide builds in thickness it also increases
in resistance. Theoretically it should reach a point of balance
where the amperage is zero. In real life it is not worth waiting for
it to happen and the precision is not necessary. In the RMS studio
where we do thousands of anodized small parts we use both voltage and
amperage as a guide to the correct color. Say we set the voltage to
62 volts at 3 amps. When the power is applied the voltage starts at
zero and slowly increases. As it reaches the preset 62 volts the
amperage begins to drop. We do not wait for it to zero. We have
determined that the color we are matching is at 62 volts and 0.1
amps. When the numbers match that we stop. Waiting for it to read
zero amperage would actually produce a slightly higher voltage color.
We could wait, but time is money. Whether you wait or not has nothing
to do with anodizing at higher voltages.
There is no right way. This is an art medium in our hands and for
our purposes. It is your eye that sees when the color is "right."
Trust in yourself and your art will speak for you.