I did a whole rundown using Gimp 2.10 before noticing OP is using Gimp 2.8
@ WATERMELON The printing company will be using a specific color profile something.icc You need to find out what they are using. If you know the name most can be downloaded from the internet.
EDIT: oops, missed a probable required preliminary
View menu -> Display Filters: Add Colour Proof to the Active filters
![Image](https://i.imgur.com/NW29eCo.jpg)
If you open the image and go to Edit -> Preferences -> Color Management
Start off with:
1. Mode Of Operation: No management - this is what you will probably see when editing
2. The CMYK icc profile that the printing company uses.
3. Change the out of gamut colors to something that stands out
![Image](https://i.imgur.com/RWbSWDm.jpg)
4. Now change Mode of Operation to Print Simulation.
![Image](https://i.imgur.com/HWfQogO.jpg)
In CMYK bright colours, blues and greens become muted. Nothing you can do about this.
Gimp is a RGB editor and really you should be using a CMYK editor such as Krita. However a useful site (free) to convert RGB to CMYK is (of course)
https://www.rgb2cmyk.org/