You can change the color calibration using Colorsync Utility. If it looks like the iMac screen is warmer than your other two monitors, it probably is.
Everyone would be better off if they forgot the Colorsync utility was ever written. That thing is a piece of garbage, and it ignores a lot of fundamental problems. Overall what makes you think it would be better than the stock profile? Color profiles break really really easily. What would make anyone think that tweaking a couple sliders is a better approach?
I opened it up and this seems way too complicated for me; can someone point to a tutorial? Thanks!
It's not complicated. It's just useless as it merely attempts to adjust the output gamma and a couple other things in a terrible manner. The problem is that neutral is a relative thing. These displays most likely correlate with different color temperatures and gamuts. They will probably never match. You may get them reasonably close via profiling them to a common target that is within reasonable reach for all three assuming you own a recent model colorimeter. Otherwise give up now. Using the colorsync utility typically results in worse profiles than the stock ones. What I don't understand with the preloaded stock ones is why Apple takes such an odd approach to optimizing them. I've looked up how they handle input/output characteristics, and I can't figure out why they chose to do it that way.
If you do decide to get a colorimeter, it has to be one of the newer ones. Spyder 4 and the colormunki display or i1 display pro are the only ones that do a reasonably good job on most of the newer displays on the market. I'd just set everything to native and profile on both first, then adjust the target if they're still too far apart. Make sure the displays have at least 30 minutes to warm up prior to profiling/calibration. Give the colorimeter at least 10. The colorimeters need to warm up at least somewhat. They also vary an inconsequential amount due to display heat. You just have to understand that trying to match them precisely means trying to achieve a common target between devices. Colorsync is not even remotely appropriate for that, and given the complexity of display profiles, it's just a terrible idea. You're going for something visually close regardless, as you won't get every value from 0-255 across 3 channels to a tolerance of < 1 Delta E across 3 different displays. There is just way too much bad advice on display adjustments on this forum.
Take a picture?
The iMac screen has neutral whites, unless it's defective.
If you're used to the cool blue screens of your other monitors, then it will, of course seem warm.
Calling them neutral isn't a great way to look at it. They should be reasonably neutral visually. They're still cooler than D65 when measured. Overall I don't buy into the idea of "neutral". Usually if I say visually neutral, I mean that without a meaningful amount of other light, most people will not perceive a cast. Shooting it with a camera is biased though, as sensors vary and it matters how you set the white balance.
Buy a monitor calibration device then calibrate them all to get them close. Dead easy to use. Go for a Spyder Express 4 or ColorMunki Display as a basic device. Dead easy to use. Install the software, plug it in and follow the on screen instructions. Takes about 10 minutes a screen.
I'm biased in favor of the colormunki display, not to be confused with the low end spectrophotometer. My only concern would be that I haven't reviewed the basic software that comes with that unit. i1 profiler comes with the i1 display pro. It's okay, but the documentation could be better. Do remember this doesn't guarantee a perfect match. These are different display types with different characteristics, even just setting up a good profile can take care of a lot of little weird problems.