As we mostly use CMYK inks for our various print processes, we recommend designing in CMYK or converting colors to this color mode so you can have a more accurate perception of how your design will print. This also allows you to have greater control over the color values; as well as manage the consistency of the color values.
If your file contains other colors such as Pantone (spot colors) or RGB, they will be converted to CMYK mode (as required) during the proofing stage of your order. However as there are several conversion methods, the CMYK values produced may not be consistent, or the best match for your color. For this reason, we highly recommend that the conversion to CMYK is done to the original design file before uploading.