So maybe moving the moon (if that is what Regina and Emma are doing) is the breaking of a Curse and brings daylight to NL. And that causes TL rainbow???
Yes, that is exactly what I think is happening. NL being in perpetual darkness signifies it being under a “shadow of doubt” or a curse. Bringing daylight signifies the breaking of that curse. As we saw when Emma broken Henry’s sleeping curse with a maternal true love kiss, we know the combined power of Emma and Regina’s love for Henry could be powerful enough to break any curse, resulting in rainbow beams shooting out as the sun returns to NL.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy