Not just for printers any more - this article introduces Wide Gamut CMYK, AKA WGCMYK, the colorspace designed with RGB'ers in mind. 

The Wide Gamut CMYK profile presented in this article is designed to allow RGB images to be edited in CMYK mode, then converted back to RGB with a minimal loss of color saturation. It does this by simulating incredibly pure inks. This profile is not suitable for printing CMYK images.

There are color correction goodies in CMYK-land, and I hope this article will make it clear that CMYK may be used without bruising even the purest, most saturated RGB colors. 

In my previous article RGB to CMYK is OK, I presented a counter to the myth that converting an RGB photograph to CMYK ruins your colors, and illustrated this with several example images.  Now I'd like to show another approach to using the CMYK color space, that does not involve any loss of color, even for images containing extremely saturated colors.

Just to review - the following pure red, green, and blue colors lose just a bit of their purity after being converted from RGB to CMYK ( and back again to RGB for viewing here! ).

As you can see, pure red, green, and blue lose a bit of their zip when converted to CMYK.  Blue in particular comes out looking very scruffy indeed.

I have a surprise for people who read the previous article: images whose colors suffer noticeably on being converted from RGB to CMYK while not exactly common, are not exactly rare either.  The reason for this is simple: the cyan dye is a wimp, and as a result we cannot do a good job on saturated blues in a CMYK image.  Check out the following image of a Fringed Polygala, kindly provided by Ken La Rose:

As you can see, the flower's nice looking purple glow is lost after a single round of conversion from RGB to CMYK and back again.  Why?  As I mentioned, the blue channel has relatively large areas of completely "blown out" blue, and when the color conversion is done into CMYK mode, blues are toned down a bit to simulate the limitations of current printer technology. 

This loss of blue saturation is has caused many skilled and knowledgeable PhotoShoppers to write off CMYK mode as a bad deal.  If you read Usenet, you will read many stern warnings in the PhotoShop news groups warning you never to use CMYK mode, ever, unless you intend to print your images commercially in a book, magazine, or newspaper.

This web page shows a way that even RGB'ers - people  whose images will only be displayed on a monitor, can use CMYK without sacrificing even their precious  lapis lazuli blue colors, or any other colors for that matter. 

How is this possible if CMYK has the limitation in cyan dye just mentioned?  Turns out you can use PhotoShop to create a CMYK profile intended to represent the purest of pure cyan, magenta, and yellow dyes!  Good news, I've done this for you, so all you need to do is download the appropriate copy of the wide gamut CMYK profile, and install it on your system with your other profiles.

Once you have installed the file, use the Edit>Color Settings menu item to bring up your color settings, click on the CMYK entry, and select "Wide Gamut CMYK".  I recommend using "Relative Colorimetric" intent, with both black point compensation and dither enabled.

Now you are set to Repeat the RGB->CMYK experiment, and you will see something more like this:

Cool, huh?  I hope this profile will allow RGB'ers to venture into CMYK space to try some tricks with the extra K channel that is now available.  

One thing to remember: due to the optical properties of cyan ink, your printer always prints in real CMYK, and working in WGCMYK does not change that fact.  So, just as in RGB mode,  your most saturated blues will still look duller than the beautiful ones you see on some of your photographs.

Now, the big question, and arguably one that should have been answered first: exactly why would you want to use CMYK as a color space for color correction?  One quick answer is you now have four knobs (CMYK) to twiddle instead of three (RGB).