How to avoid avocado in sushi rolls from darkening?
You can try dipping it into acidulated water (water with acid added, either lemon juice or vinegar, or even a powdered vitamin c tablet).  This should delay the browning, but I am not sure it will last long enough to hold the rolls overnight.

You will have the additional problem that the nori will begin to soften overnight.

See also:  What&#39;s the best way to keep sushi fresh for the following day?