I Can't Get My Book Cover Right

[Zorba]

Hi and thanks (very much) in advance.
I am making a book cover for a paperback book -- I am using Affinity Publisher, and I also have Affinity photo (and Designer, too).
There is a photo (from Unsplash, free to use) that I want to use on the cover (the photo is attached). It is a blue sky with a cloud -- and the blue is so bright and wonderful.
It's a jpg file, but the problem is, when I convert to CMYK (which is required by the printing company) -- the blue sky fades, and loses all its magic.
Is this what always happens when JPG goes to CMYK ? ... Or is there something that I don't know about Affinity Publisher (or Affinity Photo) --
is there some trick or technique to keep the CMYK blue looking bright like the jpg blue sky ?
Thanks again ... from Zorba

Edited March 22, 2021 by Zorba

[kaffeeundsalz]

59 minutes ago, Zorba said:

Is this what always happens when JPG goes to CMYK ?

It's what always happens when RGB goes to CMYK. These are the two color spaces you're converting between. It has nothing to do with the file format. And yes, the loss of bright, saturated blue tones is expected when converting to CMYK. These colors are simply not part of the color model because physics. Your only way around this is the use of spot colors in print, but this is probably too complicated and Affinity lacks proper support for spot color channels.

In your case, I would first try to add an HSL adjustment and shift the master hue about 20 degrees clockwise before converting. This will significantly alter the sky color towards cyan, but it should at least bring back the bright colorful appearance of the original image.