Color Mode: RGB vs CMYK

As a giclée printer, I often see confusion about color mode.   For this discussion I will focus on RGB and CMYK, although there are other color modes with different functions and uses.

RGB (red, green and blue) refers to the additive or “light” color process.  When these three colors of light are combined; white light results. The absence of these colors of light appears as black. This is the idea behind display devices, like your TV or Computer Monitor.

CMYK (cyan, magenta, and yellow) are subtractive colors. These colors reflect light back. Theoretically when combined the three colors should produce black. Because the resulting color is not a “pure” black, printers add black (K) to the combination resulting in CMYK.

RGB refers to light (additive) while CMYK refers to pigment, inks or paint (subtractive). Because all printers, be it inkjet, laser, or offset use the subtractive process to produce color it seems reasonable that when creating files for printing one would work in the CMYK mode. This is true for traditional color printing often called offset or 4-color process. The technology behind this printing process is to create 4 separate printing plates. Each plate was originally created by photographing the image to be printed four times using a filter for each color.  The resulting separations representing red, green and blue were then reversed to create the negatives for its color opposite.  For example red is opposite cyan on the color wheel, because cyan is the result of mixing blue and green light and the absence of red light.

Although the inkjet printers and devices that are used today by many professional printers use pigmented inks that are CMYK or variations of CMYK, the file mode format should be RGB.  This is because the files are converted by the print driver or RIP (raster image processor, fancy name for print drivers), into CMYK.  If the files are not in the RGB mode, this process still happens but sometimes with unwanted results because CMYK has a smaller color gamut.

So should you produce files in RGB mode or CMYK mode? The answer is it depends. If your files will be sent to a printer who uses an offset or 4-color process press then you should convert your files to CMYK.  If your printer uses a digital process and prints on an inkjet printer, you should keep your files in RGB mode. If you are unsure what process your printer uses, ask.  If you want the best possible color for your image, be aware of how it will be produced.


Matching Difficult Color

soft proof screen shot- Art work by Joe Moynihan, used by permission

One of the most difficult colors to reproduce is a bright Azure blue (a bright mix of blue and cyan). I have tried many methods and color corrections and still have been unsuccessful in close color reproduction.   As you can see in the side by side comparison screen shot, the colors look great on the screen.  But when I print them, the inks and media drop the intensity of the blues and cyans becoming faded, muddied and flat.

 To visualize the effect of print I use a method called “soft proofing”.  I apply a soft (or visual proof) by going to the “view” menu in Photoshop and choosing “proof set-up”.  From that drop down menu I choose “custom” and then select the paper and printer profiles I have for my printer. (In this case it is Epson 9800 Dual Black inks Breathing  Color Chrome White Canvas)  The bottom screen shot shows the results on both of my test prints.  Before I even print them, I know the image on the Left will not even come close to the original despite it being closer in color on the computer screen.

 Because I have had this issue before with this color, I have managed to find a way to reproduce it coming fairly close. I usually work in RGB color mode and send all files to my printer in RGB. (Image>Mode>RGB color).  As this is the color mode of the computer and the color mode that the printer reads I always work in this mode.  However, achieving this brilliant azure blue in RGB is not possible. So I convert the color mode to Lab color and  I am able to create the correct color on screen and from the printer. Color correction can usually be done by the numbers but when it comes to colors that are out of gamut it sometimes takes intuitive reasoning and experimentation.  Don’t be afraid to experiment in Photoshop.  It is sometimes the only way to get the effect you want