Does your piece contain more than black color? Files that contain color text, artwork, or images must be saved in four color “CMYK” (Cyan, Magenta, Yellow, Black) color mode. Files submitted in the RGB or Pantone color mode will need to be converted to CMYK. Note that converting RGB to CMYK will cause a color shift as they are not the same color spectrum.
Is your piece black and white? Files that are black and white must have all artwork or images in Grayscale color mode.