Getting the appropriate color details upfront is very important to setting up your artwork correctly.
If you select kraft, clear, or metallic material, and are looking to create special effects, it will require the use of white on your artwork. Some examples of special effects would be:
Adding a window to your bag
Making a part of your design appear as metallic
Adding vibrant colors to kraft material
If you would like to learn more - view White Ink printing/setup.