Color management - or lack of same plus potential gamut issues when you work in color modes such as CMYK. Take a pick.
You're in color proof - RGB into CMYK. Just press Ctrl/Cmd + Y, once.
(I edited my reply to make it more complete)
It seems my initial response to your inquiry was not sufficient so I will give it another try with more detail.
You chose, for reasons not mentioned, to prepare your work in CMYK mode. As a result you have available to you a smaller spectrum of colors from which to choose than if you worked in RGB – to which all the colors presented in the Color Picker are accessible. To alert you to the CMYK color choice limitation, you may call on the command View > Gamut Warning, which covers with gray those colors not normally obtainable with the CMYK ink set. The comparison is shown below.
This smaller spectrum of colors is referred to by lithographers in part of the country as the PIG: the printing ink gamut. Note in the illustration that the color chosen in RGB is outside the CMYK PIG.
The reason for this difference is that the CMY printing inks, which act as filters are is not perfect. They reflect their assigned colors (which you see), but also reflect hues within the ink pigment (and other components such as vehicles and driers) that, if perfect, they would absorb.
Cyan ink should absorb all the R of the RGB light falling on it. It doesn’t.
Magenta ink should absorb all the G of the RGB light falling on it. It doesn’t.
Yellow ink should absorb all the B of the RGB light falling on it. It almost does.
How does each rate? The Cyan is the worst of the three. Magenta takes the middle spot, and Yellow is excellent.
Since you work in CMYK for your illustrations, I assume you have been in a printing plant. If you would like visual proof of Cyan’s limitation, the next time you visit your printer, ask him to take an ink knife and draw down Cyan straight out of the can onto a sheet of white paper. You will see that as the color gets lighter, it shifts toward gray because its color reflecting impurities neutralize the color.
A handy reminder of color reflection (which you see) and absorption (which you don’t see) is displayed in the Info panel: Each color in the CMY segment of the Info panel displays the color name (C, M, Y) and in its adjacent RGB panel it its right it displays the color it absorbs.
That is why, when working in CMYK:
Increasing C reduces R cast in an image
Increasing M reduces G cast in an image and
Increasing Y reduces B cast in an image
Reducing C increases R cast in an image
Reducing M increases G cast in an image and
Reducing Y increases Y cast in an image
In a offset litho plant running a CMYK job, when the subject matter requires colors outside the PIG, and client demand and budget allow, an additional “spot color”, usually specified by its PMS number, is added to the pressrun much the same way as a spot press varnish may be added.
A seemingly logical option with goals such as yours is to change the Cyan in the CMYK set to a PMS color that would be more helpful in producing the desired Blue, for example. True, it would, but unfortunately it would do that at the price of color accuracy of other color mixtures in the image that include the “modified” cyan. Even shifting Y’s lemon yellow to chrome yellow (Caterpillar tractor color) will affect the image. So a balanced set of CMYK colors such as those presented in the Photoshop list, is a prudent choice.
Now let’s get to your illustration. The blue you would like to appear in your CMYK image is not obtainable with normal printing inks. It is outside the PIG. Photoshop shows you the nearest achievable color. Barring the option to shoulder the cost of additional PMS spot colors, you must live with that limitation.
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Jackson Masterson wrote:
... but when i did Ctrl+Y it just showed this. Please help.
That's because you're fully in CMYK mode now, as well as as CMYK preview (since you hit Ctrl+Y). See how it says it twice on the tab where the image name is?
Go to Image>Mode> and change CMYK to RGB. It should take you out of CMYK preview as well. If not, hit Ctrl+Y again.