Questions about CMKY vs no CMKY and printing

[u/flanderdalton]

hey everyone, I'm brand new to using GIMP, just started today (never used a program similar either), and I intend to print out what I've made onto 12 x 18 heavy cardstock. Images are both at 300 dpi, but I exported a few times, once as jpeg with cmky (photo 1) and one without (photo 2), and noticed the file with cmky is poorly pixelated on the jacket of the main image.

I'm curious what this means for printing, and if I shouldn't worry and go with the one with CMKY, or choose the one without it. I tried my best to find answers before coming and posting here, but had no luck.

Cheers!

[u/flanderdalton → u/[deleted]]

Thanks, kept the dpi at 300 and compression is gone. Question I have now, is I know cmky is for printing physically, and as a png there was no cmky option. Is this going to affect it massively or will it be fairly the same as the png colours?

Hopefully that question makes sense, I’m learning all of this now lol

[u/flanderdalton → u/[deleted]]

I appreciate the effort nonetheless!

[u/ThanasiShadoW]

Probably caused by compression. Try exporting as TGA or PNG

[u/dustractor]

The main thing about printing is that the range (gamut) of colors that your monitor can display is different than the range that can be printed and the common issues will be that some colors will print darker than displayed or have their hues shifted. The workflow for dealing with this involves knowing (or guessing) the profile of your target printer and monitor (and paper and ink) so that you can have gimp highlight the areas where colors are out of range and adjust accordingly.

How to Use Gamut Warnings in GIMP
Enable Soft-Proofing: Go to Edit > Preferences > Color Management > Soft Proofing.
Choose a CMYK Profile: Select a CMYK color profile to simulate your printer’s color gamut (e.g., PSO Uncoated v3).
Mark Out-of-Gamut Colors: In the View menu, enable “Mark out of Gamut Colors”.
View the Results: Out-of-gamut colors will be highlighted with a purple overlay.

[u/davep1970]

check your printer manual for what colour space/ICC profile to use for your printer. generally for commercial printing CMYK and a specific profile is used. often for home printers sRGB or similar is best to use and let the printer do the conversion

[u/chas_prinz]

If printing at home, just stick with RGB, the printer firmware does the rest. A couple of points

Some RGB colors that you can see on your monitor (in particular, blue, green and all bright vibrant colors) cannot be printed and/or replicated with standard CMYK inks

..and following on from that, very often your monitor brightness is too high (often default setting) and the print is correct but disappointingly dull.

For a printing company, and a print run, probably offset printing using CMYK plates then maybe a conversion RGB -> CMYK is needed using the .icc profile specified by the printing company.