Most printing services require CMYK color encoding, which is not natively supported by Inkscape. Because of that, you will need some third-party solution.
Firstly, I have tried Inkscpae extension, ExportPDFCMYK, but unfortunately it turned out not working in my setting (Ubuntu 14.04).
But then, I have came across ghostscript-based solution, that works great! Simply export your project into PDF in Inkscape and then execute:
gs -dSAFER -dBATCH -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite \
-sColorConversionStrategy=CMYK -dProcessColorModel=/DeviceCMYK \
Inspired by zeroset.