If Rienne dies at the same time as one or more other multicolored creatures you control, its last ability triggers for each of those creatures.

If an effect causes a non-multicolored creature to become multicolored and that creature dies, the non-multicolored card will return to its owner's hand.

If a multicolored creature you control leaves the graveyard after it dies but before the delayed triggered ability resolves, it will not return to its owner's hand.

If Rienne leaves the battlefield after its ability has triggered but before the next end step, its delayed triggered ability will still return the card.