What, exact, version of AE (down to the build number) are you using on these machines?
How are you comparing the color between them? What are your rendering settings?
On Mac it's AE CS6 and on Windows, AE CC 2014. I use the default render settings. Best settings, lossless. I compare their hex codes and it's the same.
Which Mac OS? Is it 10.11.5?
Which Windows OS? Is it 10?
Is CS6 version 11.0.4?
Is CC 2014 188.8.131.52?
The default render settings are likely an MOV file on Mac and an AVI on Windows, so the file type is different anyway, but there shouldn't be a noticeable difference.
How are you checking these hex codes? Are you bringing the rendered file back into After Effects and hovering your mouse over an area and looking at the info panel or are you doing something else?
Are you doing it on the same machine that made the file or are you bringing both files onto one machine? And, if so, which machine?