"Export to PDF" sounds like a solved problem until the output has to go to an actual printer and come back as a physical book. Screen PDFs and print PDFs are different artefacts with different requirements.
RGB is not CMYK
Browsers work in RGB. Printers work in CMYK. The conversion is lossy and not symmetric — saturated RGB colours, particularly bright blues and greens, have no CMYK equivalent and come back visibly duller.
Nothing in the browser warns you. The proof looks right on screen and wrong on paper, which is an expensive way to discover the problem.
Practical mitigation: design within a CMYK-safe palette from the start rather than converting at the end. Muted colours survive the round trip; neon does not.
