Turns out the culprit was Eye Adaptation and the Post Processing Volume’s EV100 settings under Lens > Exposure.
Here you can see my original setup. With Min EV100 and Max EV100 both set to 1,0, the effect is as described in the original post, with Scene Color Materials showing up as being too dark.
Here is a screenshot with those settings turned off (default auto-exposure settings). Now the SceneColor Materials look too bright.
Finally, after some tweaking, you almost can’t even see the Materials using SceneColor anymore, as expected. The EV100 settings I settled on are Min EV100 of -0,3 and Max EV100 of 0,0.
I am still a total newbie at everything Lighting- and Post-Processing-related in Unreal Engine 4, and many of these settings not only confuse me, but drive me into a corner that makes me think I’ll never get the project to look like what I want it to. But sometimes it’s just as simple as finding the right two dials, and setting them appropriately. I’m sure I’ll get more of a hang of it with time… or I’ll end up hiring someone who has the proper know-how. 
Thanks for your attention. I hope this helps someone else in the future.