OK, I got puzzled by this issue and made some experiments.
The perception of text color seems to be influenced by screen resolution and antialiasing of character edges.
Of course, monitor settings count here. However, using the same monitor with fixed settings connected to the same hardware BUT different operating systems results in different perceptions of text color and text contrast wrt. background. (As a side note, text contrast is very important for accessibility, and some countries mandate specific regulations of this matter for public administration sites.)
The issues my colleague had seem to be related to default font rendering in Linux distributions. An injection of Infinality gives a much better (but not perfect) visual experience!
Given how much Medium cares about quality, maybe it would make sense to have a guide or list of hints to get the best visual experience from it.