RGB to CMYK

This is a bit of a hard one to explain fully, so please bear with me.

The total range of any color model is known as the gamut. In general terms, CMYK and RGB color models have different gamuts. In addition, RGB has a larger gamut than CMYK.

In general terms, CMYK's range is limited in producing some warm reds/oranges, bright greens and light browns compared to the RGB color range. Deep browns, dark reds and purples, and deep browns are harder to reproduce in the RGB color range without mottling/dirtying up the view onscreen. Especially at smaller sizes online or in print.

Choosing the "right color" in print depends on a large degree on the paper (or other substrate) you're printing on. Coated paper stocks can reproduce brighter colors and hold more ink than uncoated or color-shaded substrates like newsprint. It also depends to some degree on the inks you use on press, the quality of the press itself and the talents of the press operator(s).

If it's a high-end printing job, your best bet is to get with your customer service rep and discuss your concerns. If the job is critical, quality proofs or test runs will provide better answers than just winging it. If it's low-end color, like color copies or low-res screen printing, or you're getting a wide range of printing output, like creating advertising for multiple newspapers, magazines and direct mail pieces, determining precise color reproduction will be a heck of a lot harder. But to test low-end/multiple output situations, I go to my nearby office supply store and get color copies to see how it'll look with the most indifferent printing I can find.

So in short, if you're doing a high end print job with one vendor, ask your print rep and run proofs/test sheets to make your best informed choice. If you're doing low-end or multiple vendor jobs, run a test page at your local color copy shop, pick your best choice and cross your fingers. Either way, aim before you fire and realize you're taking your best shot.

Hope this helps,

Randy

Converters are never the same - each converter is going to use a different formula to change RGB to CMYK.

The Adobe conversion engine is different calculations than say the Pantone formula.

Trying to match CMYK on screen to print is a much larger conversation.

If color is this critical to you, I would invest in a Pantone swatch library and rely on those numbers to match the printed samples.

https://www.pantone.com/color-bridge-guide-set-coated-uncoated

Now the problem is that, if I go the the properties of the color swatch, and choose "RGB" instead of "CMYK" for the Color mode, enter FF5050 and then re-change the Color mode to CMYK, I get a totally different color, ie (0, 79, 60, 0).

Why is that ?

It is because the online converter does not use an ICC profile like the Fogra 39 to convert the RGB color to CMYK.

and which one do I have to choose to get the right color in print ?

Why don't you directly choose a CMYK color? It will be more simple.