What the tool reads along the edges of a photo
White background checker opens the photo in the browser and reads the colour of every pixel in a thin band along its four edges; the middle of the picture is not read. Each edge pixel is tested three ways: whether it is exactly pure white, whether it lies within a few levels of white, and whether it is exactly one particular light grey, the grey that Marketplace image resizer offers as a padding colour.
The verdict has three forms: pure white, when almost every edge pixel is exactly pure white; near white but not pure, when almost every edge pixel is close to white; and not white otherwise. Each states the share of edge pixels that are exactly pure white. Under it stand the share close to white, the share of the exact light grey and the average edge colour as red, green and blue values. There is no marketplace to choose.
When to measure the edges and when to use another tool
This tool fits a packshot that is meant to stand on pure white, when the question is how far the background is from that value: a near white verdict with an average colour a little below white shows how much the background has to be lifted. It does not look for the product, so it has nothing to say about fill, cut-off edges or shadows. Product photo checker separates the product from the background and reports those for one chosen marketplace.
The two tools can read the same photo differently. Here, a product, a model or a shadow that reaches the edge of the frame lies inside the band and counts as not white, even when the background is pure white; Product photo checker classes the background it has separated from the product. Image size checker reads no colours at all, and AI image label checker reads only the markers inside the file. Remove background can deliver the cut-out product on solid white, and that result can be measured here again.
How to read the figures and what they do not show
Near white with a high share close to white usually points to light or processing: the background was lit or saved a little below white, and the average edge colour shows by how much and whether it leans towards a colour. Not white with a low share can mean a grey or coloured ground, but also a product, a border or a shadow in the band.
The tool measures the edges only. A grey patch or a shadow in the middle of the picture that does not reach the band is not read, and transparency is not read either. A pure white verdict is a measurement of the edge band, not the approval of a marketplace, and the channel's rule page remains the last reference for which image has to be pure white.