RGB is a subtractive color space made up of red, blue and green respectively. Together they form white. By subtracting intensity, a wide range of hues are visible. The less light there is, the darker the image. Hence the absence of all color is black (or the color of the display). RGB colors are used in digital cameras, on computer displays and televisions.
RGB offers a wider color range than can be reproduced with CMYK inks. The CMYK range is not just extended in all directions. Instead, more colors are available for certain hues. This means there are RGB hues not visible in CMYK and visa versa. As a result, a photo containing bright hues may appear darker after converting to the CMYK color space. It is not so much an issue when converting a CMYK photo to RGB because the latter can display a wider color range. Transparency (absence of pixels) may be retained within an RGB file by means of an Alpha channel. Sometimes such a file is referenced as RGBA.
The gamut of RGB colors is affected by the model used to define colors. There are actually different RGB spaces. The most common used by computer displays, digital cameras, and the default for Adobe Creative Suite is sRGB because it reproduces brighter colors and approximates the color gamut of various computer displays. For this reason, it used for images displayed on the Internet. However, Adobe RGB (1998) improves upon the later gamut (~25% shadow, ~50% midtone, and ~75% highlight) primarily in cyan-greens so it is more commonly used by graphic designers for print. (Note: An earlier Adobe RGB color space with slightly more cyan-green but less in every other color extremity exists so the improved one is distinguished by the year.)
During the process converting an RGB image to CMYK, software must be instructed what to do with the out-of-gamut hues. Interpreting what value of red, green, or blue to use must also be established. This is accomplished by means of ICC profiles that define the original color space more accurately.
If a photo will be printed using the four-color CMYK process, it is best to convert images to this color space before submitting to the printer in order get a better idea of how they will look when printed, identify out-of-gamut hues and reduce ink buildup from RGB conversions in black areas. Inkjet printers that include 8 to 12 inks are capable of reproducing a larger range of colors within the RGB color gamut. Therefore, photos may not need to be converted to CMYK for inkjet output.