The first step to defining a custom field is telling Drupal that our module provides a field. This is done by implementing hook_field_info(), hook_field_formatter_info(), and hook_field_widget_info(). The combination of which provides some basic information about our field including a label, description, default settings, and basic information about how the field will be formatted and what widgets can be used for data collection.
In this lesson we'll implement the basics for the following hooks:
Doing so will allow us to enable our module and see our new field type appear in the list of available fields to add to a content type. The field won't do much beyond that yet, but it's a good start towards telling Drupal about our custom RGB field type.
If you want to just follow along and look at the already written code you can grab a copy in the companion files section of this page and use that to follow along.