Have you ever adjusted at an image based on how it looks on your computer screen only to find that it prints differently on press? You can always tell how well an image will print on press by looking at it’s numbers. What numbers? The numbers that I’m referring to here are the percent values for each color that are measured using most any image editing program.
For decades color separators and scanner operators have been evaluating color by the numbers to determine it’s correctness. With the advent of desktop scanners and automated image editing software the art of color evaluation is being lost. Creating an image is easier, but knowing what to look for to establish color accuracy is disappearing.
Evaluating color by the numbers is a far more reliable and consistent method for making decisions on how well an image will print. We are going to take a look at some important color concepts, number values, and relationships to help improve our understanding. Once you know what to look for, evaluating your images will become a whole lot easier.
Info Palette
In Adobe® Photoshop® you can use the Info palette to determine an image’s exact color values to assist you in editing it’s color. You can rely on the Info palette values much more so than with your eyes or computer monitor.
Color Space
The CMYK color space represents the printed process colors Cyan, Magenta, Yellow, and Black (K), which are expressed in percentages from 0 to 100. The number combination in CMYK order will like this (87,13,67,12). The RBG color space represents the visible light colors Red, Green and Blue, which are expressed in the pixel depth values from 0 to 255. There number combination in RGB order will look like this (25,87,215).
All images are created in the RGB color space, and many image editing tools are only available in the RGB color space. However, all color images are printed in the CMYK color space. It doesn’t matter which color space you use for evaluating and editing colors, as long as you convert your images to the CMYK space in the end.
The Color Ball
Let’s quickly look at how colors are built to better understand how to assess our numbers. We start by imagining a colored football, with white at one tip and black at the other. Then consider a cylinder connecting the two tips thru the center of the football, going from white to black with all the gray values in between. Then going around the parameter of the football are the primary colors Red, Green, and Blue.
For the cleanest (most saturated) colors move to the outside and center of your color football. To make a color dirtier, move to center of the ball (which decreases the primary color and increases the opposite color). To make colors lighter or darker, simply move to the corresponding tips. While all gray colors will reside in the center core of the ball with equal amounts of Red, Green, and Blue. Congratulations, you now know the fundamentals of how to evaluate and adjust colors.
Opposing Colors
To make a color dirtier or cleaner we use the opposite color. On a color wheel you can observer that the primary RGB colors are exactly opposite the CMY colors. What this simply demonstrates is that to achieve one color you can’t have any of the opposing color. For example; to make a image more Red you need to remove Cyan. To make an image less Green, you would add more Magenta. To make a perfect Blue, you need to have no Yellow. To make any color muddier just add more of the opposite color in the spectrum.
Color Wheel
Black is actually not considered a color, in that it only makes colors darker. It does help to add definition and contrast to an image. But for the purposes of printing color images black is only a supporting value. We use Black sparingly, and it should not be the dominant value in any CMYK number combinations.
Endpoints
As odd as it may sound, the first step to evaluating and adjusting any color image is to look first at it’s gray values. Start by defining your white and black endpoints -- to establish your gray core. Then your remaining gray values should fall in line.
There should always be some values (or halftone dots on press) for the whitest point of an image. This helps to make a visual distinction between the white of a page and the image itself. It is a graphical technique used to help define the edge of an image. Common white points are RGB (244,244,244) or (241,241,241), and CMYK (3,2,2,0) or (4,3,3,0).
Then on the other end of the spectrum, the darkest gray values should never be over 95%. This leaves a little room for halftone dot gain on press (plugging). For images with a lot of dark areas, too much plugging on press will result in flat looking areas without any depth. Common black points are RGB (5,5,5) or (10,10,10), and CMYK (95,87,87,65) -- Black can be any value less that Magenta and Yellow.
Balanced Color
A balanced color image is one that has good gray values. Remember the color football, the one with all the gray values at the core of the ball. The basis for all accurate color images are with it’s core gray values. If an image’s gray values are balanced, then all it’s colors will fall into place like an umbrella around it’s handle. After establishing your endpoints, this is the next most important color consideration.
In the RGB color space, gray is defined by equal amounts of each color (0,0,0; 87,87,87; 255,255,255). With CMYK the gray values are a little different. Magenta and Yellow are always equal and Cyan is about 10% more (3,2,2,0; 55,50,50,31; 95,87,87,65). Once again it is important to remember that Black is only a supporting value.
Maximum Density
An image’s density is a value obtained by adding all four CMYK values together. As far as color images are concerned, a total density of no more than 350% would be ideal. The total density for our black point example (95,87,87,65) is 334%. The issue here has to do with the layers of ink coverage printed on a sheet of paper.
Too much ink will not dry properly, and transfer from the top of one sheet to the backside of the sheet on top of it in a stack. This is not to say that you can’t have an image with density values of more than 350%. But it is important to note that large areas with high density values can present problems on press.
Saturated Color
The brightest red, green, and blue colors in the RGB color space are the very ones that do not convert well to CMYK for printing. Or I should say, not as well as what you see on your computer monitor. We can, however, determine the best possible color combinations by looking at the numbers. In the RGB color space they are; Red (255,0,0), Green (0,255,0), and Blue (0,0,255).
Again, please keep in mind that the purest, absolute Red, Green, and Blue colors in an RGB image, will not convert well to the CMYK equivalents. If you have an image that has very clean/bright colors, you may want to convert it first to the CMYK color space before making any evaluations or adjustments.
Basically the equivalent saturated Red, Green, and Blue colors in the CMYK color space are; Red (0,100,100,0), Green (100,0,100,0), and Blue (100,100,0,0).
Memory Colors
Memory colors are those that you see often in life like blue skies, green grass, and even flesh tones. These type of colors are ones that you remember and can tell when they look right. For these colors there are no absolute values, there are many values for each of them. Therefore, instead of trying to remember every possible value for these colors, let’s just remember that the values are in relation to each other.
For example if you evaluate come flesh tone values, in the CMYK color space, you will notice that Magenta is the main color. Cyan is always a little higher then Magenta -- about 10% higher, and Yellow is less than Magenta -- about half of Magenta. If you look at a blue sky color (on a clear day) you will notice that it is almost completely Cyan, a little bit of Magenta, and Yellow is practically non-existent.
Learning and remembering a few of the basic memory color relationships are always helpful in determining if your image is in balance. If colors shift from there normal relationships you can easily tell, for example, that you end up with purple skies and anemic looking people.
Process Color Guides
The easiest method for determining color values and their correctness in an image is by using the PANTONE 4-color process guide. Every color you could every want will have a color swatch and the CMYK color values available in this guide. It is a handy reference tool, and an absolute guide for evaluating whether or not your image colors have the correct value ratios.
I understand that looking a color values and their relationships to each other can get a bit confusing at first. But with a little practice, evaluation of an image’s color can become quit easy. The more you look for some of the main color number combinations and grayscale values in your images, the quicker and more familiar you will become with color by the numbers. This will provide you with the knowledge you need to evaluate how well any image will print on press.
