Dark acrylic (or glass), is used for three purposes. The first is contrast enhancement. CRT's make pretty good blacks, but still not perfect. But by the time you cut down the amount of light being produced at black levels by 85%, it's virtually a dead black, which is very nice. While the rest of the colors aren't as blinding as they would be otherwise, the increase in contrast makes them "pop" more without looking overdriven. It's also more comfortable to look at, especially in subdued lighting environments.
The second is the lack of a need for CRT shrouding of any sort. For example, I have a 27" RGB monitor wedged into a cabinet that was originally designed for a 19" monitor. Very little room for a shroud, and it would probably look silly if I cut one into the narrow strips that might have a chance of fitting in there. With the dark acrylic, there is no indicator of a monitor edge at all.....which brings me to...
The third reason is that on a 27" horizontally mounted monitor, a 19" vertical game (there were lots of them) appears at almost actual size. But the thing that always spoiled the look of those games was the fact that they looked "pasted" into the center of a monitor that was turned the wrong way. With the dark acrylic, there is no indication of monitor orientation. 19" vertical games just appear in the center of the view window of the cab, at pretty much actual size.
Going with lighter material will give you benefit number one, albeit to a far lesser extent, but does nothing for reasons two or three. So the choice becomes quite simple, when you understand why those who go dark decided to do so.
RandyT (a former optician who pioneered the use of dark view window material in arcade cabinets many years ago....)