RGB is made up of three color channels (red, green and blue) and this is what’s displayed on monitors. CMYK is composed of four channels (cyan, magenta, yellow and black) and this is what’s used in the four color printing process.
In the digital scrapbooking world, most papers are created and saved in CMYK, and element and alphas – anything with an alpha channel (transparent background) is saved as RGB. Why? The standard format for saving elements is as a PNG file, and PNG doesn’t support CMYK.
If you’re creating something that will eventually be printed, my advice is to create it in CMYK. The file will be larger, but you won’t get to the end and convert it and find that the rich beautiful blue you had in your artwork is now a muddy steel color.
The RGB spectrum has a lot more colors than CMYK does, and some colors won’t convert. Open a CMYK document in Photoshop and open the color picker and select a color. You can see in this example I picked a nice bright red – in the top right of the color picker window a swatch is displayed which shows the last color I had on the bottom and the color I picked on the top.
Great…now I want a nice brilliant blue. But what’s this? See that little triangle with the exclamation mark? That means that the color I picked is out of the CMYK gamut. The color under the little cube is the color I picked, and the color under the triangle is what I’m going to get. It’s important to watch for this when working on something that you’re eventually going to print, or you may get a nasty surprise when it’s already printed!
Some Photoshop filters will only work in RGB mode, but you can convert your artwork to RGB, apply the filter, and then convert back to CMYK. Photoshop always asks if you want to flatten the layers when converting, because it affects layer compositing. You should always choose NOT to flatten the image if you’re going to convert to a different color mode and then convert back. If you’re working in RGB and are going to convert to CMYK for a final output, it’s best to flatten the image when converting and retain the blending options you have set.
TIP: A nifty trick is to work in RGB mode with CMYK preview turned on. That way all the filters are available to you, your file size is smaller, and you can see approximately what your colors will look like once you convert your final file to CMYK.
Use the key combination CTRL+Y (Windows) or COMMAND+Y (Mac) to toggle CMYK preview on or off.