The holodeck basically manipulates light. Therefore it isn't hard to theorize that it somehow is able to break up the light spectrum to where only black and white is shown, and not the rest of the color spectrum that the human eye normally sees. It's also possible that the holodeck would materialize something over their eyes, like a contact lens, small enough not to be noticed, that would somehow block out the rest of the visible light spectrum. If the holodeck can generate wearable clothing, it's not that hard to imagine it could generate something like this also.Best guess? energy fields projected in the same manner as the holograms would modify the wavelengths of light passing through them. the holograms themselves were already monochromatic, so filtering out one or two participants' colors wouldn't be quite as challenging.
canon, there is no responce. "it just can"
When the Doctor first visited the holodeck with the Captain Proton program running, he was in full color, and Kim asked the computer to "adjust the Doctor's spectral frequency" (VOY: Night). While the Doc is a hologram, it implies that the holodeck can adjust what a person percieves on the holodeck, probably much the way that it can modify a person's clothes (though it seems many people prefer to dress up in costime anyway), or when it made Seven of Nine's implants disappear in the episode "Human Error".