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I have an image that is CMYK and I have tried to convert it to RGB so that I can edit the colour of the image. However it won't let me work in 32 bit and asks me to convert the image to 16 or 8 bit to be able to use the paint bucket tool.

If I use it without doing this I get a different colour to the colour I wish the image to be.

Can anyone help please?

Thanks J

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migrated from photo.stackexchange.com Dec 20 '11 at 18:30

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Could you post the image, or part of it along with the colours you're trying to apply and what actually happens to the image? (is it the whole image that changes colour or just the colour you're applying that's wrong?) –  drfrogsplat Dec 3 '10 at 0:48
    
CMYK and RGB are different color spaces. This is to be expected whenever converting between the two. –  DA01 Dec 20 '11 at 19:09
    
How do I convert an image to 8 bit RGB color in Photoshop CS2? –  user3309 Jan 4 '12 at 9:11
    
@Jodie maybe this will help > youtube.com/watch?v=LhIy5KafE3E –  Flavius Frantz Jan 4 '12 at 12:56
    
Alike question with usable answer: graphicdesign.stackexchange.com/q/5104/14041 –  e-sushi Jul 23 '13 at 2:29
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2 Answers 2

The colour shift is probably not related to the bit depth (8 or 16), but more likely to the gamut of RGB vs CMYK.

As far as I'm aware, when Photoshop refers to 8-bit or 16-bit colour, it's talking about each channel, so 16 bits is plenty for individual colour channels in RGB, and won't be the cause of changing colours (if it was 8-bits shared between the three channels, probably using a colour palette, it'd be ugly, but at least in my experience, Photoshop talks in bits per channel).

The CMYK gamut is different to the RGB gamut, and it's probably just the conversion between the two that causes the colour change. This article on gamut is probably a good place to start understanding the concept. The basic problem is that the colour spectrum isn't really just combinations of R, G and B, and projected/printed colours can only cover a part of the actual spectrum that our eyes can see.

The problem with converting between CMYK and RGB (in either direction) is that there are some colours that can be represented in one, but not in the other. The strongest/brightest reds, greens and blues that can be shown on an RGB device or represented in an RGB format simply cannot be created from a CMYK device/format. Likewise, the strongest cyans, magentas and yellows from the CMYK gamut will not be accurately reproducible on an RGB device or in an RGB format.

When you display a CMYK image on your RGB monitor, you're effectively limiting yourself to only the colours which both gamuts can reproduce (the monitor cannot go beyond its physical RGB limitations, and Photoshop is algorithmically limiting the colours to the CMYK gamut).

So if you're trying to apply a strong red, green or blue to the image, it simply cannot exist in the CMYK format and so won't show up as you expect on your screen, since Photoshop is converting the RGB colour you've picked to the nearest CMYK colour available. It's worth noting that if you print the image, it'll appear much like the CMYK version (you cannot print the strongest Reds, Greens and Blues your screen can show).

If the image is only for on-screen viewing, then convert to RGB. If it's for printing (which I would assume is why its in CMYK format?), then you're not going to be able to print those colours outside the CMYK gamut anyway (unless you can print with Pantone colours instead).

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I think you may just be running into a problem with the two color models. A CMYK image is 32bit because there are four separate color channels to represent. The RGB model only has three color channels to represent. When you convert from a 32bit CMYK image to an RGB image, the two common options are 16bit or 8bit, as outside of floating-point HDR images (which is an unlikely option here), there really aren't any 32bit integer RGB image formats (excluding those with an alpha channel of course, which would be RGBA.)

Its difficult to explain this, other than to just say that there is no "direct" conversion from a four-channel CMYK imate into a four-channel RGB image. The conversion is a fairly complex mathematical process involving ICM that is translating colors from one color model (the subtractive CMYK model) to another color model (the additive RGB model). I can't really say exactly how much distinct color information your CMYK image may contain, however I would be doubtful that it contains as much color accuracy as a 16bit RGB image can represent. You are probably safe to convert into an 8bit RGB image, however if you really want to make sure you preserve the maximum amount of color accuracy, I would convert into 16bit RGB.

There is no 32bit integer RGB format I know of that you can convert into, or at least, not one that Photoshop supports. HDR images are 32bit floating point, however there is no direct translation from a CMYK integer format into an HDR floating point format. Even if there was, a 32fbit HDR image can portray a range of tones and colors far beyond anything reproducible in CMYK print, and you might end up with some undesirable artifacts if you managed to find a way to convert between the two.

As for the color differences, you will need to make sure that the images are assigned the appropriate color profiles. ICM is involved in conversions between color models, so making sure that your source image has a valid and appropriate color profile assigned to it that properly maps the colors it contains will be important in preserving color information when converting to RGB. It is highly unlikely that you will be able to get 100% accurate representation of the CMYK colors in RGB due to significant differences in those two color models, and the differences in the color profiles assigned to both images. There will most likely be some color shift between the two. You may have to do some pre- and post-processing in both images to set proper white/black points, adjust contrast and saturation in RGB, etc. to get a correct final image out of the conversion.

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