Anyone know the answer to this?
Supposedly, the security cameras that they use for spotting cheats can see through red fabric. I find this claim highly dubious.
Ummmm…
Bulls? 
Superstition? Being “in the red” is usually a bad thing when dealing with money. Black, on the other hand, is a perfectly acceptable color to wear using this rationale.
Well, according to the Gaming Horoscopes at this promotional Vegas website, wearing red just might pay off…
So I guess these guys never heard of it. Couldn’t find much else with some quick searches.
Not an answer, but for those wondering where the question came from, it’s the title of a book by casino security expert George Joseph.
…in which, I believe, is made the claim that security cameras can see through red fabric, but as I said, I think it is highly unlikely; there isn’t just one kind of red fabric and there isn’t just one kind of security camera, to name but two problems with the claim.
I would think that (if true) would be a reason for wearing red dresses in the casino.
While the security staff is concentration on looking thru the woman’s red dress, her companion is unobserved and can be doing things to increase his winnings!
There are imaging systems which can, for example, image through layers of clothing. But those imaging systems usually use terrahertz or lower frequencies, and are fairly expensive, so they’re unlikely choices for casino surveillance when normal visible or near-IR security cameras will suffice. To my knowledge, conventional visible/IR imaging systems are not capable of creating clear images of people “underneath their clothes” – especially not in the visible, and generally not in the IR (though there was an interesting fiasco with some of the Sony Handycams with the Nightshot feature, in which the images created were “indistinct” at best). If any peeping tom activity is going on in casinos, it would most likely be based on the Nightshot idea.
The terrahertz systems work on the principle that the wavelength is longer (on the order of a millimeter) than the absorbing medium (in this case clothing, which is also mildly porous on a scale much smaller than a wavelength), so the waves can penetrate the clothing, but not the skin. This isn’t the case with visible light, since clothing is much thicker than the wavelength of visible radiation, and highly absorbing/emitting. However, terrahertz imaging systems would work regardless of clothing color. Here are a few articles about it (if you’re willing to forgive the first one for referring to radio waves as “ultra-high frequency” – they’re actually relatively low frequency): link1 link2
In summary, the “red dress” story is probably just an urban legend.