By default, PostPilot uses the RGB color space in our color picker. RGB was created for colors on digital monitor screens which use pixels.
Some RGB colors displayed on a monitor cannot be printed on paper -- mostly really bright greens, blues, and pinks. That's why in printing, the CMYK color space is used.
Below you can see the color spectrums of both RGB and CMYK:
When you view a preview or proof of your design in PostPilot you are viewing a CMYK version. Colors may still vary slightly based on your monitor settings. PostPilot generates high-resolution PDF files and color profiles to be an accurate representation of the specific CMYK colors in your design.
If you want to really geek out on color spaces, check out Wikipedia here.