i once designed a red logo with inkscape which only knows rgb. i set the color to 255 0 0 (RGB) and created a pdf which i sent to a digital prining service. they printed the document and all my logos where rose rather than red.
in order to fix that, back at that time, i used adobe illustrator, opened the pdf and simply converted the color scheme for my logo from rgb to cymk.. i had it printed again and it came out red, which is what i wanted.
today i don't have a windows machine anymore and i don't subscribe to adobe software, so i need a new method to convert an rgb pdf to cymk for printing.
luckily there is ghostscript, which can do this fairly easily:
gs -dSAFER -dBATCH -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite -sColorConversionStrategy=CMYK -dProcessColorModel=/DeviceCMYK -sOutputFile=banner_4c.pdf banner.pdf
keep in mind that this will auto convert rgb to cymk which in a way isn't really possible, so this is not the right solution if you need to have an exact color in the end.. in that case i think “scribus” might help.