Here's a rough example of what I meant earlier about "starting with a 'lighter' grey...". And sorry if this is long and boring for some.
Let's say we start with this excellent rendering.....
For all intents and purposes, it's created as sort of the reference as to what level of 'grey' we'd like the bodysuit part as. But it's also done with basically neutral/full ambient lighting...in this case from behind/through on a computer monitor. Almost as if he's standing in a surgery room with full-level light all around him. But if this were in real 3D, we'd be applying light TO it...let's say a basic spotlight-type of light, which I've simulated as 3D layers in After Effects with a virtual light shining onto it as if it were a real object. I've compared starting with the original (above), and one with a lighter base shade of grey (bottom)...I didn't change/adjust the levels on the images of Batman, I only changed the level of light shining on him.
With the original...although the 'grey' part looks 'just right' in full light, it basically becomes another shade of black as you go with darker lighting conditions, as may be appropriate for a Batman film. It coudl easily register to the viewer as black, and perhaps a different texture from the cape/cowl/gloves if it can even be distinguished at all...especially in movement.
The second/bottom version starts off with a grey that may be too bright in full/higher lighting conditions...but as we darken the lighting towards our overall look like we did with the original, the 'grey' part remains more distinguishable than the 'black', and pretty-much equates the original grey that we liked by the time we hit our target lighting. It might still be too light of a grey, but for demonstration purposes you can hopefully see how it reacts in comparison.
This is assuming, of course, that the '50%' light is the general kind of light level that Batman would be in for most of his time onscreen. If the bottom suit suddenly steps into a brighter, more direct light, then the grey would suddenly look brighter, obviously. So that's why so much time goes into lighting/shot/material design fro something like this...and why a single rendering cant quite tell the whole story as to 'what it should look like'. It's also why sometimes a production snapshot can make something look so crappy, as opposed to how it'll eventually be shot through the cinema cameras, then timed and adjusted for final print....and why going 'all-black' from the outset can be easier to work with.