Roll20 provides an FX Playground for experimenting with the custom effects (https://wiki.roll20.net/fxplayground/
), but it just gives you a text area that you have to manually edit JSON in. This script replaces that box with a full UI, including being able to load in the built-in effects and colours.
The first image shows the FX Playground's default UI, and the second shows what this script replaces it with.
The documentation for custom effects in Roll20 is here: https://help.roll20.net/hc/en-us/articles/360037258714-F-X-Tool