RGB vs CMYK for printing
Short answer
RGB is an additive colour model used by screens; CMYK is a subtractive model used by print. Some colours that a screen can show cannot be reproduced with printing inks, so converting RGB artwork to CMYK will change those colours — the conversion makes the shift explicit rather than causing it.
Why the difference matters in practice
The colours most affected are saturated blues, bright greens and vivid oranges. On screen they are produced by emitted light; on paper they have to be produced by ink absorbing light, and the achievable range is smaller. A customer who approved a design on a monitor has approved something the press cannot fully match.
Supplied sRGB R0 G102 B255 Output CMYK C85 M60 Y0 K0 Visible shift: yes · blue desaturates
The conversion depends on the profile
There is no single correct CMYK value for an RGB colour. The result depends on the destination profile, which describes the behaviour of a specific combination of press, ink and substrate, and on the rendering intent, which decides how out-of-gamut colours are brought inside the range. Converting without a named profile is guessing.
What to tell the customer
Not that their file is wrong, but that some colours will shift and by roughly how much. Showing the measurement is more useful than describing the theory. If exact colour matters — a brand colour on packaging — the answer is usually a spot colour rather than a better conversion.
How Prepta handles it
Prepta converts using the output profile named in your production profile, records both the source and destination profiles in the audit trail, and reports colours that shift noticeably instead of converting them silently. Where a profile requires spot colour, process colour is escalated rather than approximated.