Discolouration of Sushi Rice
well, for starters, sushi should be eaten fresh, but if needs must:

1) the cucumber has some alkaloids that we usually take out when preparing (my mom call it the "evil" of the cucumber). I cut the "tops" of the cucumber and use a circular motion to friction the sliced tops on the main cucumber "body" and let capillary action do the rest;
2) also for freshness and crunchiness, after you slice the cucumber, sprinkle it with some salt and let osmosis work for ~15 minutes. Squeeze the water out and roll your sushi. If you do this, the sushi rice (which is seasoned) won't draw out the water in the cucumber and take with it the coloring

I've never had this discoloration problem, and this are the 2 main things that I know that both me and my mom do that are not exactly common when preparing sushi.