To create a custom filter or modify existing one, create a json file and place it in the following path of your datapack: data/<pack>/exposure/filter/<filter>.json
To attach filter to a camera, you need to add the item to #exposure:filters tag.
If you want a filter to use a custom shader, you'll need to create a resourcepack as well.

Example Datapack

Contents of json file have several properties:

• predicate: ItemPredicate, same as in advancements. Used to define valid items for this filter.
• shader: ResourceLocation of a shader.
• attachment_texture: ResourceLocation of a custom texture for showing in Attachments menu. (optional)
• attachment_tint: String representation of a hex color. RRGGBB. Used to tint attachment_texture. (optional)*

Example of a spider eye filter:

{
  "predicate": {
    "items": "minecraft:spider_eye"
  },
  "shader": "minecraft:shaders/post/spider.json",
  "attachment_texture": "example:textures/gui/filter/spider_eye.png",
  "attachment_tint": "FFBBAA"
}

Note: attachments_texture in above example points to a texture added by Example Resourcepack