The Rainbow project was created as an open source initiative to build a comprehensive web-based content management system using Microsoft's ASP.NET technology. Rainbow's core architecture provides user authentication, internationalization (there are currently 14 supported languages), and a customizable roles-based authorization system that allows content authoring to be safely delegated to multiple team members who need little or no knowledge of HTML. Rainbow also supports a two-step approval process where content is first posted to a staging server and then migrated to the production server after approval. The Rainbow architecture provides an easy mechanism to add new functionality as portal 'modules'. 45 modules are included in the standard release, including an eStore, XML news feeds, articles, surveys, threaded discussions, document management, user-defined lists, and more.

The initial Rainbow design, based on Microsoft's IBuySpy (IBS) portal kit, has been significantly enhanced and improved by a global team of volunteers. Most IBS users either quickly outgrow its simple functionality, or become frustrated finding and applying the various patches and enhancements available on the net. Migration to Rainbow is a natural evolution.