This page explains how to forge rainbow weapons and armor in Chrono Cross: The Radical Dreamers Edition.
Rainbow weapons and armor are the best equipment found in the game, in terms of raw stats. For most characters, they are the best equipment full stop, although armor-wise, the plates stolen from the dragons have much better functionality (being able to absorb Elemental damage).
Acquiring these equipment needs a lot of prep work though.
How to Obtain the Master Hammer
To begin with, you need to find a hammer that’s powerful enough to create your rainbow gear.
For that purpose, you will need to complete the sidequest to restore the Home World version of Marbule. This can be done after receiving the mission to seek the dragon relics.
After Marbule has been restored, keep playing until Serge gains his body back. When that happens, return to Marbule (Home World). You’ll discover that people have started returning to their daily lives. Head to the back cavern where the Black Dragon would be. One of the folks there will sell the Master Hammer for 10,000G.
What to Do With the Master Hammer
Just the hammer itself isn’t enough–you need somebody skilled enough use it. If you haven’t done so, visit the Smithy in Termina (Home World) and recruit Zappa. With Zappa in your party, hop between worlds and visit the same Smithy in Another World. Speak to Zappa there and your Zappa will initiate a conversation.
From here on, you can craft rainbow equipment (usually prefixed by “Spectral”, “Prism” or other rainbow-y words) at this Smithy or when you use the Smith Spirit key item at places where you can save.
Where to Find Materials for Rainbow Equipment
The final step is to gather the raw materials for your rainbow weapons/armor. Because it’s a fairly involved process (which part of this isn’t!?), we’ve split the information into the following two pages:
Create a rainbow weapon.
Create a piece of rainbow armor.