As samfisher suggests, OpenCV is not that hard to get working on the Mac, and Core Image is a great Cocoa framework for doing GPU-accelerated image processing. I'm working on porting my GPUImage framework from iOS to the Mac, and it's entirely geared around making accelerated image processing easy to work with, but unfortunately that isn't working right now.
If you're just getting started on the Mac, one tool that I can point out which you might overlook is Quartz Composer. You have to download the separate Graphics Tools package from Apple's developer site to install Quartz Composer, because it's no longer shipped with Xcode.
Quartz Composer is a graphical development tool that lets you drag and drop modules, connect inputs and outputs, and do rapid development of some fairly interesting things. One task it's great for is doing rapid prototyping of image processing, either using Core Image or OpenGL shaders. I've even heard of people using OpenCV with this using custom patches. You can easily connect an image or camera source into a filter chain, then edit the filters and see live updates as you work on them, without requiring a compile-run cycle.
If you want some sample QC projects to play with, I have a couple of them linked from this article I wrote a couple of years ago. They both do the same color-based object tracking, with one using Core Image and the other OpenGL shaders. You can dig into that and play around to see how that works, without having to get too far into writing any code.