A colorspace is a way that computers can use math to describe a range of colors within an image. The two most popular colorspaces are CMYK and RGB. For digital display projects only (web, online, kiosk, e-book, etc.) RGB is the proper default—it is, after all, the “natural” color definition for pixel-based displays.
When designing a project for print, though, you should use CMYK. Why? Because all printers use CMYK colors to put the image on paper, from home inkjets to professional laser/digital types to industrial offset presses.
Let us explain the difference between RGB and CMYK in a nutshell.
RGB = Red, Green, Blue. Think Online Color
RGB stands for Red, Green, Blue. If you are showing an image on any screen (such as Phone, TV, Computer Monitor, etc...), you should make sure your image is RGB. RGB typically has more vibrant colors and can display more colors than CMYK.
RGB is known as an additive model. The colors are added together to make up what you see on the screen. Pixels on a screen are one of those three colors. Light is projected through the pixels blending the colors on the eye's retina to create the desired colors.
RGB is all about seeing the light. Computer screens show color in images, text, and designs with different red, green, and blue light combinations. This is where RGB comes from. Therefore, anything designed for a screen – from smartwatches to a jumbotron – should be designed in RGB color mode.
CMYK = Cyan, Magenta, Yellow, Black. Think Printed Color
The name CMYK comes from the four colors that make up the model: cyan, magenta, yellow, and key. Key represents the color black. Since “B” is taken by “blue” in the RGB model, the last letter of the word “black” is used instead of the first.
CMYK is a subtractive color model and works exactly opposite from an additive color model (RGB,) it works by partially or entirely masking certain colors from white. So instead of adding light to achieve color, CMYK is using ink to subtract brightness from white. Therefore, Cyan, Magenta, and Yellow combined is black - the absence of brightness.