Arturo,
Fundamentally, the range of colours is narrower in (plain) CMYK than in RGB, (especially) including brighter/more stunning colours.
You can see a bit about the fundamental differences,
https://duckduckgo.com/?q=rgb+vs+cmyk+gamut&t=ftsa&atb=v320-1&ia=web
If you wish consistent colours, the safe way, as recommended in the old days, is to start out with CMYK colours.
But maybe things have moved so much into the bright(er) world of (RGB) screens that the appearance of (plain CMYK) print has a dwindling importance, so that many will settle for a certain resemblance, maybe using extended gamut printing for more important prints.
You can read a bit about extended gamut printing,
https://duckduckgo.com/?q=extended+gamut+printing&t=ftsa&atb=v320-1&ia=web
Edit: And what Ton said while I was still at it.
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