For CMYK output jobs, we always recommend converting the colours to CMYK mode.
This allows you to have greater control over the colour values; as well as manage the consistency of the colour values.
Colours 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 colour. For this reason, we highly recommend that the conversion to CMYK is done to the original design file before uploading.