This document is generated with PdfGenerateConfig.PdfXConformance = PdfXConformance.X1a - the strictest PDF/X level, requiring CMYK-only content and forbidding live transparency. It carries a caller-supplied CMYK ICC OutputIntent.
The heading above and this paragraph's un-set default text color are both plain RGB in the source HTML - X1a converts an achromatic (gray/black) RGB color to CMYK deterministically via ColorOptions.BlackGeneration (UseTrueBlack here) rather than rejecting it, since that has an exact, lossless ink mapping unlike an arbitrary hue.
A genuinely chromatic color must be authored with device-cmyk() under X1a - color: orange here would throw at generation time instead.