Imagine all the different visible colors, the millions upon millions of all different shades and hues. Now, picture them as a three-dimensional sphere, with white on the top most point, moving to pastels and light colors until the "equator" which has the most saturated pure colors, and then shading towards black at the bottom most point. If you start spinning your sphere, like a globe, on its White/Black axis, you would see the colors of the rainbow along the equator morph from red to orange to yellow, to green, cyan, blue purple, magenta, back to red, etc. as it turns.
Okay, you can stop spinning around. Got your sphere in your mind? We are going to pretend that this is an accurate 3D model of all the visible colors.
In reality, the 3D image of all visible color is not nearly as perfect or even very spherical, but reality is usually messy and this is just a thought exercise!
Now, inside of your sphere of all the visible colors of the world is another sphere, a smaller subset of colors. This is what we see on our computer monitors and televisions. This is the RGB color set. It can't re-create all the colors we can see with our eyes, but there are still millions of colors it can produce.
Inside that sphere is another smaller sphere. This one represents all of the colors that can be printed in ink, on paper. In our perfect imaginings, this sphere is smaller, and it will represent an average CMYK color space, for example all the colors that our printing presses here at Lorraine Press can reproduce with the normal four colors of ink.
This is a very simplified way of thinking about color spaces. But it's also a helpful way of imagining what is happening to your image during the printing process without getting into the nitty gritty and usually complicated details of color spaces. Each of step of capturing and then printing an image uses a different color space: seeing something colorful, taking a picture of it with your digital camera, looking at it on your monitor, and printing it out on your printer.
If you are interested in what a CMYK color gamut looks like in reality (the smallest sphere you imagined), click here to go to a blog post by Gordon Pritchard. The post is talking about spot colors, but the 3D image of a CMYK gamut is a good one.
Now, imperfect, messy reality demands that each monitor, digital camera, and printing device has its own unique color subset, or sphere, its own subset of possible colors it's able to reproduce. Some are RGB and some CMYK. And also in reality CMYK color spaces can even intrude outside parts of the RGB color space in certain colors.
Each imagined sphere, or color space, also called a color gamut, represents the complete subset of colors that can be accurately reproduced in that color space. It is a measured fact that the visible color gamut is much larger than the RGB gamut which is in general larger than the CMYK gamut. And this is a reality we all must live with, and it can get messy.
Here is another (two dimensional) way to look at it: (graph courtesy of Wikipedia)
The gray area represents the entire visible spectrum. And the colored triangle is what an average monitor can show. (Even though each monitor is slightly different, the technologies we use can only display a small portion of visible colors.) It may seem limiting, but your eyes are usually happy with what they see.
It gets a little more complicated when you add in the CMYK color space, but here is another graph which adds the typical CMYK color space with the typical RGB values inside of the visible spectrum: (graph courtesy of BU, who pulled it from Jackson, Richard, MacDonald, Lindsay and Freeman, Ken, Computer Generated Color, John Wiley & Sons, Ltd, West Sussex, England 1994, page 221)
The CMYK area is the pentagram looking shape represented by the dotted line. Note the areas of green (at the top of the RGB triangle on the graph) and blue (the bottom left corner of the RGB triangle) that the CMYK color space cuts through. These areas involve colors that you can readily see on your monitor, but are impossible to reproduce in regular CMYK printing.
These colors are called out of gamut for the CMYK color space. There is no way to make the CMYK gamut print those colors. Therefore, when viewing a photograph on your computer screen, you should keep in mind that it is possible that there are places in your photo that CMYK won't be able to reproduce. Adobe Photoshop has a tool that can show where these places are in an image.
In Photoshop, on a Mac or PC, have your image open, then click on View, then go down to Gamut Warning. Or if you like keyboard shortcuts, hold down command and shift and the "Y" key to toggle the Warning off and on. Here is a regular RGB image, opened up in Photoshop:
Note the grey speckles within the black circles. These are areas that are out of gamut. This is just a warning. And it doesn't change anything in your image. It just turns on and off. Now you are probably wondering what to do about the warnings. We'll talk about that in Color Spaces - Part 2.
In the next blog post, Color Spaces Part 2, since this one is already way too long, (kudos to anyone who made it this far!) I'll explain more about color gamuts, color conversions, and why knowing about color gamuts is important to those wanting to print in color.
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