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:

f=poster.pdf
gs -dSAFER -dBATCH -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite \
  -sColorConversionStrategy=CMYK -dProcessColorModel=/DeviceCMYK \
  -sOutputFile=${f%????}_cmyk.pdf $f

Inspired by zeroset.