|If you think you know all you should know about CMYK and RGB, then answer this elementary question: Which one is subtractive color -- and why? Well, even if you can answer that question, it is still good to be thoroughly grounded in the basics. So let’s take a short look at some color stuff that should be as ingrained in you as the last episode of "Friends"- - perhaps not as exciting but I can say, at the very least, say more colorful.
I am sure that you are familiar with dots and know that if you fill a given area on a white sheet of paper with 50% black dots, allowing 50% of the white paper to show, you will have a shade of gray. You can achieve 99 shades of gray by using dot concentrations of 1% to 99%. Obviously, when you reach 100% you have solid black and 0% would be pure white- -- providing the paper is pure white.
In the same manner, you can get 99 shades of the specific process blue we call cyan in the same manner. The same is true for magenta, a specific process red, and also for a specific process yellow we call interestingly enough, “yellow.” Mixing the various percentages of CMY will yield about 1 million colors (100 x 100 x 100 = 1,000,000). At this point you may be asking about the K or black color component of CMYK. Well, you don’t need black or K to make black. 100% of CMY makes black. It does this by subtracting all the color present in a white paper.
White is the presence of all color and black is the absence of color. In print we start off with all color in the white paper to which we add our ink. We are, in effect, subtracting color as we add the various blends of CMYK. Thus CMYK is subtractive color. Hopefully now that you know the answer to my original question you will still read on because we are about to get even more colorful.
If we can make black with 100% each of CMY why do we need K or black? First try to imagine printing black 7-point type by registering CMY. You might succeed in getting a 3D effect by giving each and every reader a set of colored 3D spectacles. But in all practicality, other than a severe registration test for your printer, you would be wise to print black type with just black. Second, if your image has a solid black background, imagine how much ink you’re pouring onto the paper by using solid CMY -- too much for good offset printing. Besides, you wouldn’t like the black you get from 100% each of CMY since it’s somewhat muddy and not as rich or deep as you would want for a good crisp image. Moreover, by adding black you are getting better grays and significantly adding to the amount of colors you can achieve.
So if CMYK is subtractive color, the color of the paper -- your starting point -- is a key component of achieving good color and that is the color of the paper. Whether the color is more yellow-white or blue-white affects the outcome of the color you are trying to achieve. For example, a creamy-white paper can produce warmer colors, while a blue-white paper might yield a colder or crisper result. The same is true for black, as black can become contaminated with other tones like such as red or blue. Quite often, the printer does this intentionally. By putting 10% cyan under the black you can get a richer, deeper black. Conversely, most good printers will remove any magenta because a black background that has a red cast may have an affect on the contrast and richness of the colors in the foreground image.
When you turn your computer off, you’re probably are looking at a black screen. Thus, if CMYK is subtractive color because you’re starting off with all colors, red, green and blue (RGB) is additive color because you’re starting off with no color, a black screen, and adding color. Unlike the shades of colors in CMYK, which are varied using dots, RGB color shades are varied by using levels of brightness from the electronic source: a cathode ray tube (CRT), a liquid crystal diode (LCD), or from another electronic methods. Also, unlike the percentage of dots in print that comprise 100% offering 100 variations, there are 256 (0 to 255) shades or levels of brightness for on-screen color that generate 16,777,216 (256 x 256 x 256) color possibilities. Setting each RGB color to 255 will generate white and setting each to 0 leaves black.
RGB vs. CMYK
So there you have it -- on-screen color is RGB and print is CMYK. In effect, the two are opposites since again, one is subtractive color and the other is additive color. When you manipulate and create images in the digital environment, it’s all RGB -- although programs like Adobe Photoshop allow you to convert to CMYK and effectively simulate the CMYK color space on-screen. (Photoshop is actually working behind the scenes in another color space but let’s not get into that.) The key point is that since on-screen is RGB, you have to convert to CMYK to print.
You don’t even think about this when you output to your laser or inkjet printer as it automatically converts RGB into CMYK. Why then do commercial printers make such a big deal about getting RGB files when they can easily convert RGB to CMYK? The reason is that long before digital files came along printers were using CMYK. It’s part of the fabric of their very existence and it’s how they relate to color. Moreover the conversion is not perfect. So the printer would rather the customer take the responsibility instead of being held accountable for color shifts. Another problem is the possibility of a rich black. Because RGB doesn’t have a specific black component like CMYK there’s the chance that black might convert to 100% each of CMY as opposed to K.
The RGB to CMYK conversion is getting better with less rich black problems. The advent of fast turnaround digital printing is accelerating the move to RGB workflows. In fact many print industry sages are saying that RGB will become the color standard for submitting all but the most exacting projects. Old folks like me have a great deal of trouble thinking in terms of RGB because CMYK is so ingrained in our color psyche. However, younger newcomers to our industry are more and more RGB oriented.
For now, most commercial printers want you to submit files in the CMYK color space both to both maintain tradition and to avoid the responsibility of color shift surprises in the conversion. But I agree with the many industry experts predicting that RGB will be the submission and workflow norm in the very near future.