In my installation the "QuantityPanel" FontColor is RGBColor[{0.498, 0.4196, 0.3294}]. As I said, you have to dig down to the Core style sheet to find it.

In a Mathematica notebook you can tell it's a unit because, when hovering over it, it's in a Panel with a lighter gray background and it has a Tooltip.

There is no reason that the FontColor should be so faint, or anything other than Black. That's the issue I'm raising. Why shouldn't Output look like you might see in a standard technical paper?

Actually one could use

Quantity[14, "Electronvolts"] // TraditionalForm

I still can't understand why anyone would want a light brownish grey output for an important quantity in StandardForm.

This brings up a difference in style in using Mathematica notebooks. I believe most users use them as worksheets or scratchpads. If they are going to write a document for other people they copy out results, sometimes literally by hand, to LaTex and publish a pdf document. But I claim that Mathematica notebooks, with their active and dynamic character, are so superior it would be better to write literate notebooks in the first place. And that means that significant Output would naturally be in black.

The idea that most users who are seriously using units wouldn't know what happens when you hover over them or that there would be many cases where something that looked like a unit would not be a unit are not very strong justifications. Would you claim that a physics journal should use brownish grey for any result that contained units just so the reader knew they were units and not some symbol?

If users want a faint brownish grey for Output then that's what they'll get.

I would guess the idea is that you can distinguish it from (undefined) variables:

But the choice of the actual color appears to be semi-random indeed. I see the color as RGBColor["#7f6b54"] by the way (Mac interface).