White colour

Hi everyone, Sorry for my English. I have a problem with a white colour on print. We have a industrial ink printer CMYK. We print on cardboard boxes. Cardboard boxes are brown so we couldn't print on white colour. Very often our clients while making projects im designer using white colour (because background is brown) but we couldn't print it. Do we want to blocked white colour in designer that nobody can't use it on projekt or another solutions is convert white colour to transparent. Unless is another solutions to fix my problem. Please help me, od you need morę informatorom please write to me. Dominik