code: public void setDefaultColorspace(PdfName name, PdfObject obj) {
          PageResources prs = getPageResources();
          prs.addDefaultColor(name, obj);
      }
docs: Sets the default colorspace.
  @param name the name of the colorspace. It can be <CODE>PdfName.DEFAULTGRAY</CODE>, <CODE>PdfName.DEFAULTRGB</CODE>
  or <CODE>PdfName.DEFAULTCMYK</CODE>
  @param obj the colorspace. A <CODE>null</CODE> or <CODE>PdfNull</CODE> removes any colorspace with the same name