FX is a subset of RED
Graffiti is a subset of RED
RED can do everything that can be done in FX and Graffiti plus a whole lot more. While all of these applications can use he video card for preview, they all rely on traditional scan-line rendering for the final result. Just like all other compositing and animation applications such as After Effects, Nuke etc. Also, all of these applications are cross platform - Mac and Win.
Blue is slightly different - it's really designed for real-time 3D titling and animation. It uses the video card for both preview and the final render. If you need a lot of 3D titling then Blue is a fast way to get results. Plus Blue includes vertex and pixel shaders, which makes possible some pretty cool looking title animation effects. Blue is Windows only.