We can print from either CMYK or RGB image files. However, we recommend supplying RGB files for optimal output, but if your image is already in CMYK, don't change it.
Our printers use between 8 - 12 different ink tones ranging from the usual CMYK (Cyan, Magenta, Yellow and Black) along with new colors created for photo-realistic printing such as gray, light magenta, light cyan, and more.
Our ONYX Thrive RIP is calibrated to print from both CMYK and RGB files and preserve the original colors saved in the file as close as possible. For this reason, it's best to let our specialized software handle the color on our end for the best results.
In other words, don't change your source file just to send to us - let our software do the work. If you are starting a new project RGB files tend to remain the brightest and most color accurate in large format inkjet printing.