Every one of those points could be refuted.
*doesn't become shiny.
I have a few beige keyboards that are pretty damn shiny. I also have 25-year-old black keyboards that aren't shiny.
I'd guess what he meant was: IF the surface already is shiny, where are you more likely to see it?
*not too bright, but not too dark.
That's the problem - not enough contrast.
NACK. With the right kind of lettering, you can get plenty enough contrast, though I do prefer the slightly lighter-colored keys on Cherry boards. (I have yet to find anything with better low-light visibility than old Cherry dye-sub and two-shot or Model M keys.) One of my boards with definitely insufficient contrast (lettering and spaces between keys) is a very light grey - with weak lasered lettering.
Speaking of low contrast,
picking colors can go wrong with two rather different forum themes around.
*doesn't look dirty.
Some of the dirtiest keyboards I've ever seen were beige.
Again, for the same amount of dirt, black is far worse. Very fussy. I had some fun cleaning that black FK-3002.
*easier to distinguish with two-tone beige colours.
Two-tone beige just looks like dirtier keys... or two shades of vomit. I'd prefer different high-contrast colours like:
Show Image
Ugh, '70s. Always makes you wonder where people mislaid their taste. That being said, it's not like cheap-looking silver-painted plastic, aggressive blue LEDs or PC game space ship control panels pretending to be gaming keyboards would be any better. I guess there's plenty of "WTF were they thinking?" design to be had for any era, more that's unobtrusive and gets the job done (<-- insert beige 'boards here) and finally some that really stands the test of time.
If I want '70s, I'll rather take this:
But going back to our 'boards, I think we can conclude:
1. Light-colored boards do have a few advantages when it comes to hiding shiny spots, a bit of dirt and optimum low-light visibility (with adequate lettering).
2. Two-tone setups are actually useful. They are merely unpopular because they are more expensive to produce.
3. Beige boards are unlikely to win any design prizes but they are the most common ones to implement points 1 and 2. It now is up to keyboard designers to do better.