With PdfGenerateConfig.PortableColorFilters on, brightness(), contrast(), and invert() are written as blend-mode layers (Multiply, ColorDodge, ColorBurn, Screen, Difference) instead of an ExtGState /TR transfer function, so a reader that ignores /TR - MuPDF, and the viewers built on it - still shows the filter. Still vector, not a bitmap. (docs/html-css-support.md#filters-and-blend-modes)
Peach baseline (no filter) filter: none | Peach brightness filter: brightness(1.6) | Peach contrast filter: contrast(1.8) | Peach invert filter: invert(1) |
Peach brightness below 1 filter: brightness(0.5) | Peach contrast below 1 filter: contrast(0.5) | Peach partial invert filter: invert(0.4) | Peach brightness + contrast filter: brightness(1.3) contrast(1.4) |