RGB > CMYK
Customer: texts pic of truck, I want a banner this color blue.
Me: You can’t do that.
…and so it begins.
If you know you know, if you don’t lemme help, this post is about converting RGB images to the CMYK color space for printing.
Let’s start with an image, a photo, what you’re looking at right now on your screen is an RGB image, made up of little Red, Green, and Blue LED’s, but to print your photo onto paper it needs to be converted into a CMYK print file. Color printers whether inkjet, laser or commercial sheet-fed offset presses use these 4 colors of ink, and are printed in combination, one over the top of another using semi-transparent inks.
The human eye then blends these colors together to create the illusion of an infinite range of colors, but… there are limitations, some colors cannot be reproduced with this printing process and blues are the worst. Printer manufacturers of “photographic” printers have overcome this by adding more colors, in addition to CMYK, another lighter Cyan and Magenta have been added for a total of 6 colors, this increases the spectrum of colors you can reproduce.
To print an image with a color printer, the image must go through a conversion process, your inkjet printer does it on the fly in the background, commercial printers open photos in Adobe Photoshop and “convert” photos to the CMYK color space ahead of time, some colors do not convert well, they “shift” (see truck pic above) and need to be color corrected with Photoshop to prevent surprise shifts in colors when printing on press.
To print color images on a commercial printing press, a set of color separations is needed.
The separated CMYK plates of the above photo would appear as shown below and each plate prints with it’s corresponding color.
When printed one color over another, as shown below, the look of the original photo is reproduced.
To make a banner of an un-obtainable color using CMYK colors you can create a special “plate” for printing referred to as a “spot” plate. Spot plates can also be used for varnishes and other printing effects.
Commercial printing presses.