Neutral means that the background has no colour of its own to compete with the product and nothing in it to look at. It is a wider category than pure white: a pale grey wall, a sheet of beige paper or an evenly lit studio sweep all qualify, while a patterned cloth or a busy room does not.
Marketplace rules use the term in two ways. Some require pure white for the main image and say so in exact values. Others ask only for a light, neutral ground, which leaves room for grey or beige and suits white products better, because a white product on white loses its outline.
The choice is made per channel and then held: one neutral tone across a collection lets shoppers compare products instead of backgrounds. Where one image has to serve several marketplaces, the strictest rule decides, and that is usually pure white for the main image.