Your artwork can be either CMYK or RGB, but we do encourage converting artwork assets to CMYK color mode before placing your order. This allows you to "soft-proof" your images on screen and see how the colors will look.
If you supply your images in RGB, they will be converted to CMYK for print. RGB colors may appear dull or desaturated when printed (and we're sure you don't want that!).
We've created a more in-depth explanation about the differences here: CMYK and RGB.
Please also ensure that all transparencies are flattened for best print results.