These 2 blog posts should help you.

1. Microsoft Project Baseline Percent (%) with Weighted Summary Roll-up This will show you how to set-up a custom field for baseline %
2. Microsoft Project Progress Status Traffic Light (RAG Status)This will show you how to set-up a RAG but instead of using the default % complete field as shown in the blog post, use the baseline % from #1 above for the RAG.