It is recommended you stick to black, darker colours or muted tones when printing on brown kraft paper. These colours are less likely to be affected by the colour of the paper they are being printed on and will provide a good contrast on the brown kraft paper. Other than black all colours will be affected to some extent when being printed on brown kraft paper.
Which colours should I avoid printing on kraft?
Avoid yellow and very pale or light colours as they simply don't have enough contrast to show up against the brown kraft paper. The colour wheel below should give you a good idea of which colours and tones will work best on brown kraft paper.
One thing to bear in mind with brown kraft paper: the manufacturing process may result in a variation in the colour of the sheet, with the front of the sheet being darker than the reverse.