Color space refers to the palette of colors from which images are displayed. Computer monitors have different primary colors than printers. Monitors use red, green, blue (RGB) primaries and most color printers are based on cyan, magenta, yellow, and black (abbreviated CMYK). Because of backlighting (from color displays) and other factors, RGB is capable of brighter colors than CMYK.
Because of its perceptually uniform properties, many professionals suggest using L*a*b* (more commonly called Lab; least commonly CIELAB) as intermediary conduit when converting between RGB and CMYK. The way it separates detail in the lightness channel, Lab is also suited for specific types of photo enhancement.
Adding another layer of fidelity to capturing color is bit depth. Typical "true color" photographs use 8 bits to represent each RGB color channel, totaling 24 bits per pixel with a potential for 16,777,216 colors to be represented. When 16 bits are used to to represent each RGB color channel, 281,474,976,710,656 colors can be presented.
Barring congenital visual defects, sources indicated the human eye distinguishes up to 10 million colors. (Depending on the source of the study, 100,000, 2.3 million, 7 million, and 10 million are offered as upper limits.) Therefore it appears that 8-bit color exceeds requirements and that 16-bit is significantly unnecessary. However, there are hardware limitations for representing full palettes and a desire to focus on one particular area of a photo with smooth gradations can test the limits of a color space. Diffusion dithering lessons the amount of required colors for smooth transitions.
Inkjet printers are now offering additional colors beyond CMYK to reproduce a wider range of colors. So RGB is the preferred color space for photo output. Black and white digital photos can also be submitted in their native RGB color space. They do not have to be converted to grayscale to benefit from quad-tone gray poster printing. Offset printing, with its inherent CMYK limitation is still most economical for large print quantities. Therefore, when many images will be repurposed for multiple products -- say a single large poster, 50 color laser prints and several thousand brochures -- it is often more efficient to prepare art with CMYK images.