Medium also compresses photos (not as bad as Instagram) and their file size requirements and compression/display methods seem to change on a quarterly basis.

The first step, however, is that I would double check that the issue is in fact from Medium, and not your browser or device. For example, 95% of the time I edit photos on my iPad and post to medium on my MacBook. In this way it would initially be hard to tell if its the MacBook screen or Medium that is changing how my photos look. Maybe try viewing your article photos across multiple devices, and also in the browser and app versions of Medium to determine where the color shift is coming from.

From there, I would make a fake article, and post your photos in that article to see how they display on medium, and then create a "last step" preset or mental note to boost your colors for the versions of the photos you plan to export to Medium.

I haven't noticed a drastic change in my colors when posting to Medium though, even between devices.

Hope that helps, let me know what you find in your troubleshooting.