"We don't know"? Of course we do. If you were to fly out in space and look close up with just you own eyes looking ut a window. All you would see is empty black space. The gas density is so close to a vacuum that there is so little "stuff' you would not see it.
Telescopes are dramatically larger then human eyes and can collect millions of times more light. It is not just their size but that Telescope can take very long exposures and add up the light that faal on them for hours.
So you would see blackness, but these high instruments can see the small amount of light in that darkness.
Then if you look through a small telescope from your backyard, almost everything looks white. Human color vision only works in daylight brightness, faith objects, look colorless.
It is even worse with JWST because it is looking in a range of "colors" that we can not see at all. So the data has to be translated up to what the eyes can see. Our eyes see infrared as "blackness".
"We don't know" is a very bad answer. We'd see nothing at all.
The better question is about Mars. It is red and orange but the eye adapts and after a few hours the color would fade and look more neutral. Just like our eyes adapt to light at sunset and we see it as less red than it really is.
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