Duotone is an SVG filter, and it prints in the body

The duotone effect is implemented as an SVG filter definition injected into the page, which is surprising the first time it appears in a diff of the rendered HTML.

{
  "settings": {
    "color": {
      "duotone": [
        {
          "colors": ["#101418", "#1f6feb"],
          "slug": "ink-brand",
          "name": "Ink and brand"
        }
      ]
    }
  }
}

// disable it entirely: "duotone": []

Each duotone in use adds an inline <svg> element to the body, which is inert and does affect anything asserting on the page structure — a visual regression baseline or a markup validator will notice. It applies to images and video and cannot be applied to anything else, so the palette is narrower in scope than it appears. Setting it to an empty array removes the control, which is often the right call on a site with photography.