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You probably already know that RGB means red, green, blue. Those are the colors of light shining out of your color monitor. And you probably know that CMYK represents cyan, magenta, yellow, and black, the four process printing inks. But when is each appropriate? Is one right? Is one wrong? The illustration that you see on screen is showing you a comparison of what the human eye can see--that's the external color shape--versus what your monitor can show--that's the black triangle--versus what CMYK inks can image--and that's the little white dotted area.
So you can see that there are some colors that can be shown on your monitor but can't be printed in CMYK. So what does that mean? Well, it means that you might get a little disappointment when you get your job back from the printer. But I think it's important to understand the process so that maybe you're prepared for what comes back from the printer. When you shoot a photograph with a digital camera, or you scan reflective art on a scanner, the image is stored in RGB. And you've seen the RGB color space is a fairly large one, so it can display vibrant range of colors and the CMYK Gamut is smaller.
But how can we tell what's going to happen to this image. Now we're probably losing a little bit of vibrance in video compression, but hopefully you'll still be able to see the difference. This is an RGB image, you can see in the little title tag here that it's an RGB image. How can we show ourselves on screen what might happen when it goes to the printer? Well, we have a preview function called Proof Colors. Under Proof setup if you have a custom profile from the printer, invoke that. I don't have one, so I am just going to use the default Working CMYK. Now keep in mind that your monitor is never going to perfectly match printed output.
If you calibrate and profile it may come close, but it's still light coming at you versus ink on paper. But this will at least give you a better idea of what the result might be. When I go a view, if I choose Proof Colors, I think you can see that things got a little duller. But let's do that faster, and I think you can compare better. I like to use the keyboard shortcut Command+Y on the Mac, it's Ctrl+Y on Windows, and that way I can quickly toggle between RGB and CMYK and see what the results are. So there is the CMYK, there is the RGB.
Look at the large green roll of furry stuff up the left, look at the guy's blue shirt in the front, there is CMYK, there is RGB. The green is almost florescent it's so bright, and look how much duller it looks in CMYK. It's still a good-looking picture, it still has a lot of life and color in it, but if you'll do a look preview like this you'll have a better idea what it's going look like when it comes back from the printer. Here's another image and we'll do the same little preview. Now there is CMYK, there is RGB. You can see that the bright greens show the most difference.
So if you have an image that has a lot of bright green foliage and other content that's bright green, just kind of be prepared for what's going to happen to it when it goes to the printer. One more little thing, if you want to apply filters to an image, it has to be RGB. Now I've applied some filters to this image just to make it look a little bit more interesting because I am going to use it as a background shot. But it's still RGB. If I try to do that to a CMYK image and you can see again in the little title it says that it's CMYK. If I go to Filter, I can't even choose Filter Gallery.
So if you have plans to decorate some pictures by adding Filters, you're going to have to keep them in RGB. Now in the olden days, printers insisted on designers submitting CMYK images. Those rules have relaxed a little bit. Modern imaging workflows do a better job of converting RGB to CMYK on the fly. And in fact, some digital printing devices actually have a wider range of color than offset CMYK inks, and they can do a better job if they are fed RGB content. As always, you should ask your printer which they prefer and supply them with what they need.
Even if I'm working on a job for a printer that insists that I send CMYK images, I keep my images in RGB as long as I can, because I don't want to sacrifice color range too early in the game. And as I showed you, some effect only work in RGB. So I recommend that you keep your images in RGB, don't sacrifice flexibility. If you do needed to submit CMYK images, keep your working RGB file intact and then ask the printer which color profile you should use when you convert to CMYK. Either way it's all about not being disappointed when your job comes back from the printer.
The better idea you have on screen of what your image is going to look like when it's rendered in CMYK, the better prepared you are for the results.
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