You are doing anything incorrectly. The Barcode node writes out the barcode in DeviceRGB space irrespective of the barcode color values and color type (e.g., CMYK). The net result is the CMYK barcode values get converted to RGB values in the PDF. These RGB values can/will be converted to CMYK via either the ColorManage, Optimize nodes or the DFE using a destination/CMYK profile. The conversion back to CMYK has little chance of yielding the original CMYK values specified in the Barcode node. An application such as PitStop/InDesign would allow the desired CMYK values to be written into the PDF for the barcode after the barcode was created using FFCore.