Colorista II is a great product. (So is Synthetic Aperture's bundled Color Finesse, but I prefer to stay in AE's native, color-managed workspace.) Having access to the rest of AE's toolset is wonderful and gives you tremendous control over your image.
However, while AE has many tools and a good color management system, it's a compositing and an animating system, not a color grading system. While you can produce just about any image you want with AE, its tools and workflows weren't designed with color grading in mind. It doesn't work in real time, it doesn't support more than a single still store with on-or-off A/B comparison, it has no re-conform tools, its timeline lacks NLE-style navigation tools, its built-in point tracker is poor and can't drive its native masks, it doesn't support control surfaces.
SpeedGrade is still a very new addition to the Adobe family, so it isn't (yet) completely integrated into the suite. However, it's a good product and it could be well-worth learning as Adobe continues their integration efforts.
Tero's suggestion to give Resolve Lite a spin is also very good advice. It's free, it's ubiquitous, and it's remarkably powerful.
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