A magenta to green gradient would then go through white rather than grey. A subtractive magenta-green gradient would go through black. Not sure what physical setup would produce the latter gradient. But the standard RGB (or OKLAB) gradient goes through grey rather than white or black. This type of gradient is physically created by dithering: Dithering a gradient from magenta to green, by just using these two base colors, would produce a perceptual grey in the middle. This type of color mixing is otherwise better known as alpha blending.