The good news is that you have an awesome display. The bad news is that Blender by default does not support any type of display beyond sRGB.
So before I give you the solution, it’s probably wise to wrap one’s mind around what is happening.
Your iMac uses a different set of RGB lights to emit colour. When you render something using Blender using the default configuration, it assumes your display is sRGB. That is, it doesn’t know what kind of display you have. That means that an RGB triplet sent to the display that has no compliments such as
0.0, 0.2, 0.0 will project out a different colour light on sRGB as compared to your iMac. Of course that means by extension, every single mix of lights is totally wrong. You are seeing the lights projected out though they were connected to sRGB coloured lights as opposed to your iMac’s much more saturated and different coloured lights!
The good news is that I’ve included the Display P3 colorimetry in the official Filmic Blender repository at GitHub. Change your display to it, and your rendering should appear correctly on your iMac. Just don’t save it with that display type chosen! Any issues, feel free to post.
The bad news is that Blender is still pretty woefully lame regarding pixels. That means that to properly encode an sRGB image, you would need to flip your display to sRGB and save the file, as well as ideally attach the sRGB ICC tag. It’s totally wrong looking at your display, but Blender is horribly broken and it is where we are at. Another option is to leave your render set to Display P3, save it, and assign the Display P3 ICC. Painful, but works!
Hope this helps...