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A picture that features very dark colors besides very light colors. More commonly seen on greyscale arts, but it can also be seen of coloful ones such as Danbooru post #137122.
Try to look for broad ink-like strokes, pitch black shadowing, solid black and white.
These are often results of flat color (Danbooru post #5538857) or manga-like coloring (Danbooru post #5782342), halftone may be present too (Danbooru post #5892760). Compare Danbooru post #5785450, which despite having very dark hair, has enough shading to give it more gradient and volume.
Not to be confused with spot color or color contrast.
Examples
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