The Export dialog requires you to choose between RGB and CMYK color models when you choose to export a graph (or layout) in the PDF, EPS, JPG (Mac only) or TIF format.

RGB (Red, Green, Blue) is based on projecting. Red light plus Green light plus Blue light all projected together create white.  Black is encoded as the absence of any color. Colors encoded as RGB are sometimes called spot colors.

CMYK (Cyan, Magenta, Yellow, blacK) is based on ink.  Superimpose Cyan ink plus Magenta ink plus Yellow ink, and you get black. White is encoded by the absence of any color.  Colors encoded as CMYK are sometimes called process colors.

Even though it uses one more number to encode a color, the CMYK scheme actually encodes a smaller "color space" than does RGB. That means that the appearance of some colors will change when converted from RGB to CMYK. Most noticeably, bright colors in RGB (left side of figure below) will look duller and darker in CMYK as shown in the right side of the figure below.

We recommend that you choose RGB when possible, as there will be fewer surprises. The colors will closely match what you see on the Prism screen (which internally uses RGB). Many scientific journals now request RGB.

Select CMYK only when submitting images to a journal that insists on CMYK.

Learn more about the differences between RGB and CMYK here and here.