From Granblue Fantasy Wiki
Rainbow Prisms are primarily used to uncap characters. They are needed for late stars on SR characters and intermediate stars on SSR characters.
|Uncap SR to 4★||3|
|Uncap story SR to 3★||1|
|Uncap story SR to 4★||3|
|Uncap SSR to 3★||3|
|Upgrade Revenant Weapons (step 2)||50|
|Upgrade Revenant Weapons (step 4)||150|
|Upgrade Revenant Weapons (step 6)||250|
|Uncap Trade Moon Weapons to 4★||10|
|Uncap Arcarum Summons (SR) to 1★||100|
Rainbow Prisms are found in the Violet Trial daily quest.