>> If I use a cmyk black backgound (30,30,30,100) and then export for print the background CMYK value has changed and K is not even 100!
I checked the PDF by opening it with a PDF object browser. The CMYK values of both rectangles are correctly exported:
0.60000 0.40000 0.40000 1.00000 k
0.30000 0.30000 0.30000 1.00000 k
>> It is also obvious in the pdf that it is less black than the black of the (initally rgb) images
CMYK devices cannot reach blacks with luminosity as low as RGB black. If I checks the RGB(0, 0, 0) conversion with Scribus default profiles, CMYK (30, 30, 30, 100) values are too low to reach the blackness of resulting CMYK values (75, 67, 63, 95).
Here are the luminosity expected for these different values in CIELab space:
- RGB(0, 0, 0) : 0
- CMYK(75, 67, 93, 95) : 11.67
- CMYK(60, 40, 40, 100) : 11.23
- CMYK(30, 30, 30, 100) : 12.88
In CIELab space, any color difference above 1 is expected to be noticeable. The luminosity differences between RGB(0, 0, 0) and the different CMYK values is hence quite more than noticeable.