The Chrono Cross is a item endowed with the power of the lost seventh element color, and an essential item for beating the game of the same name. To get it, go to Divine Dragon Falls in Another World once you have both the Tear Of Hate and the Tear Of Love. Place them both on the provided altars to receive the Chrono Cross. The Chrono Cross serves two purposes...
First, it allows you to save Kid from within her nightmare. I won't go into it any more than that.
Second, you can use it to defeat the final boss in short measure, and in doing so, get what's referred to as "the good ending". If you're playing the game and have the Chrono Cross equipped, you'll notice that every time you cast a spell, there's a ringing sound, and then a small sphere appears at the top of the screen.
When you defeat Aquator, notice the order in which the colors light up on pedestal. If that isn't enough, watch the order in which the next boss uses elements. When you fight the final boss, cast a string of colored elements in this sequence, and follow that up with the Chrono Cross. It's not quite as easy as that, because the final boss likes to screw up the order with his own elements. Just get a bunch of different colored low-level elements so you have something to work with.
By the way, the color sequence listed in the above write-up is wrong. I don't really think I should give it away... well, OK, how about this: I'll pipe-link it below, and you can scroll over it if you want.
final element sequence for Chrono Cross