As fans of this much-improved season will know, the Kandorians are clones of the originals and were sent to earth so that the race would survive, but they were genetically encoded NOT to have the powers they would normally have under a yellow sun. Presumably somebody figures out that the opposite would be true if the sun blazed red.
Look for the cool visual of Clark's shirt with the "S" emblem empaled on a post -- straight out of Superman's battle with Doomsday (not suggesting this has ANYTHING to do with Doomsday). One question, though: why does Lois refer to "The Blur" as the "Red and Blue Blur"?