The file rijksoverheid.gpl is an Inkscape and GIMP color palette for Rijksoverheid (Dutch government). Install these by copying the .gpl files to ~/.config/inkscape/palettes and ~/gimp-2.6/palettes or whatever directory is appropriate. Install gradients by copying the .ggr files to ~/gimp-2.6/gradients or whatever directory is appropriate. These gradients can be used in Inkscape via File / Import... or CTRL+i.
For more information on the colors included see Rijkshuisstijl Communicatiekleuren http://rijkshuisstijl.nl/ , NORA-beeldtaal http://www.e-overheid.nl/onderwerpen/e-overheid/architectuur/nora-familie/nora/ and ICTU http://ictu.nl/ .