Why do i need to print everything in 100% black?

If you print in colour, you will have your yellow security code printed on the sheet and the foil will adhere to it. When you print with 100% black this is turned off. This is why it’s important to make your black in 100% black – not a mixture of CMYK. If you print with CMYK your foil will adhere to this barcode and you will see specs of foil in a pattern all over the sheet.