To use Chroma Catalysts in Clair Obscur: Expedition 33, follow these steps:

1. Progress in the main story until you reach the Flying Waters area and complete the related story segment. This unlocks the Curator NPC at your camp

2. After defeating the Curator in a story encounter at the Flying Waters, return to your camp. The Curator will now be present in a cave to the right of the main campfire area

3. Approach and speak to the Curator at your camp. Select the "Upgrade Weapons" option to spend your Chroma Catalysts on upgrading your weapons, provided you have enough catalysts

4. Different types of Chroma Catalysts upgrade weapons to different level caps:
   • Chroma Catalyst: upgrades weapons up to level 3
   • Polished Chroma Catalyst: upgrades weapons up to level 9
   • Resplendent Chroma Catalyst: upgrades weapons up to level 19
   • Grandiose Chroma Catalyst: upgrades weapons up to level 32
   • Perfect Chroma Catalyst: upgrades weapons to max level 33

5. Later in the game, you can also upgrade weapons at any Expedition Rest Point (flag) without returning to camp

In summary, you must first unlock the Curator by advancing the story to the Flying Waters and defeating him. Then at your camp, use the Curator’s upgrade menu to apply Chroma Catalysts and enhance your weapons' levels and stats