What is black and white?
From the perspective of the physics of color, there is always color save in one condition: when there is no emission of light. Everywhere else along the infinite spectra there is color.
White's even worse. There is no point along the spectra that is "white." White is either a condition of reflected light when all light is reflected or a condition of emitted light when multiple spectra is emitted in perfect balance.
Grayscale (a pure "shade" of color between white and black) is the messiest of all - and I know this because I've mixed "gray" paint. Oh, it looks gray in the can, but put a red chair in front of the wall you painted with it and you'll see the red in the gray paint. Put blue carpet in the room and you'll see the blue in the paint.
And what you're looking for is a way to explain the lack of color, but the presence of luminescence.
You don't want physics. You want biology. You want a field that affects the perception of color by the brain. From the perspective of rationalizing the effect, you can do this in a few ways (and it's IMO a lot simpler than explaining how you changed physics...).
1. Change the way the cones in your eyes work
There are two types of photoreceptors: rods and cones.
Rods are responsible for peripheral vision, and are located outside of the central part of the retina. There are some 120 million of them, and they are responsible for night vision, because they are highly sensitive to low-intensity light. They are completely blind to high-intensity light, so they are not important for daytime vision or for visual acuity. Because they are not able to distinguish colours, they produce achromatic vision.
Cones, which vary in number from 6 to 7 million, are responsible for the visual acuity of the human eye (the ability of the eye to resolve and to pick up the minor details on an object) and for distinguishing colours. They are concentrated in the small central part of the retina known as the fovea centralis, measuring 0.3 millimetres across and devoid of rods. (Source)
Your field disables the cones in the human eye, leaving only the rods (responsible for peripheral vision and unable to detect color) as the only means to see anything. A natural consequence (if you care about that) is that your people would lose their fovea vision, meaning the world would look just a bit blurry.
2. Your field disables two of the three types of cones, making your vision monochromatic.
You have three types of cones: red, green, and blue. Your brain interprets signals from the three types in a way that's similar to how an LED display works (kinda, I'm simplifying things a lot). In other words, if the three cones see their preferred colors at the same intensity, the brain interprets that as grey. Unfortunately, it's basically impossible to guarantee with your field that all spectra would be reduced to just three. But if you removed, for example, the blue and the green cones, what you'd see is everything in red. Not shades of red, but in luminosity of red. You'd get exactly what you want... it's just not gray.
3. Your field affects the way the brain processes color signals.
This is the solution that humans understand the least, but is most likely to explain exactly what you want. The brain takes those shades of red, blue, and green perceived by the cones and creates a picture that has oranges and browns and yellows and purples and puce and all kinds of amazing colors. But if that interpretive process stopped interpreting color... what you'd see is a world of gray.
So, don't change physics. Bad Things happen when you screw around with physics. All of reality could implode! But biology! You can goof around with biology all day long and what's the worst thing that can happen?
Well... zombies... but let's ignore that.