For the best outcome in terms of printing on brown kraft paper, stick with dark colors (i.e. black) or opt for white ink. With all other colors, the outcome of your printed artwork may look duller since it's printed on a brown kraft surface.
For full color designs, we would recommend printing on white paper in order to get the best results, especially when your design is more colorful. This will ensure that the colors stand out and don't blend into the paper like with brown kraft materials.
Printing white ink on kraft paper can give your packaging an elegant, minimal look. Here are some examples for reference!