Register Graphics prints full color using the CMYK (cyan, magenta, yellow, black) process, therefore if you send files that use any other color mode such as RGB or index, they will be converted to CMYK. Converting may cause an un-expected color change in your images since RGB has a wider color gamut.
Computer monitors emit colors as RGB and scanners import images as RGB. Inked paper on the other hand, absorb or reflect light using a screened dot pattern of CMYK.