Simlest way converting from CMYK to RGB:

<?php
if ($jpeg->getImageColorspace() == \Imagick::COLORSPACE_CMYK) {
    $jpeg->transformimagecolorspace(\Imagick::COLORSPACE_SRGB);
}
?>

It is pretty work in current stable Image Magick (6.9.0-4).

When converting from CMYK to RGB using this function, the image can become inverted. To fix this, use a workaround (don't forget to download the .icc files online): 

<?php 
// don't use this (it inverts the image) 
//    $img->setImageColorspace (imagick::COLORSPACE_RGB); 

if ($img->getImageColorspace() == Imagick::COLORSPACE_CMYK) { 
   $profiles = $img->getImageProfiles('*', false); 
   // we're only interested if ICC profile(s) exist 
   $has_icc_profile = (array_search('icc', $profiles) !== false); 
   // if it doesnt have a CMYK ICC profile, we add one 
   if ($has_icc_profile === false) { 
       $icc_cmyk = file_get_contents(dirname(__FILE__).'/USWebUncoated.icc'); 
       $img->profileImage('icc', $icc_cmyk); 
       unset($icc_cmyk); 
   } 
   // then we add an RGB profile 
   $icc_rgb = file_get_contents(dirname(__FILE__).'/sRGB_v4_ICC_preference.icc'); 
   $img->profileImage('icc', $icc_rgb); 
   unset($icc_rgb); 
} 

$img->stripImage (); // this will drop down the size of the image dramatically (removes all profiles) 
?>

FYI, here is the breakdown for (int $colorspace):

Constants:
0 - UndefinedColorspace    
1 - RGBColorspace    
2 - GRAYColorspace    
3 - TransparentColorspace    
4 - OHTAColorspace    
5 - LABColorspace    
6 - XYZColorspace    
7 - YCbCrColorspace    
8 - YCCColorspace    
9 - YIQColorspace    
10 - YPbPrColorspace    
11 - YUVColorspace    
12 - CMYKColorspace    
13 - sRGBColorspace    
14 - HSBColorspace    
15 - HSLColorspace    
16 - HWBColorspace

I have a better solution for solving inverted colors on php 5.3.x than posted.  All the other solutions I found darkens the image or messes with the colors.

See below (note: my imagick object is $jpeg)

$range = $jpeg->getQuantumRange();
            $php_vs_arr = preg_split("/\./", phpversion());
            $php_vs = $php_vs_arr[0] . '.' . $php_vs_arr[1];
            if ($jpeg->getImageColorspace() == Imagick::COLORSPACE_CMYK) {
                    
                //make sure cmyk color-space is set correctly
                $jpeg->setImageColorspace(12);
                    
                // then we add an RGB profile
                $icc_rgb = file_get_contents(FRAMEWORK_PATH . DS . 'color' . DS . 'AdobeRGB1998.icc');
                $jpeg->profileImage('icc', $icc_rgb);
                unset($icc_rgb);
                    
                //set color space to rgb
                $jpeg->setImageColorspace(13);
            
                //fix gamma, hue, saturation, brightness
                if($php_vs < 5.3) {
                    //ADJUST GAMMA BY 20% for 5.2.x
                    $jpeg->levelImage(0, 2.0, $range['quantumRangeString']);
                } else {
                    //php 5.3 hack FOR INVERTED COLORS
                    $jpeg->negateImage(false, Imagick::CHANNEL_ALL);
                }
                    
            }
            $jpeg->stripImage();
            //end convert to RGB=========================|

These are the basic steps to convert an image from RGB to CMYK:

$image = new Imagick();
$image->readImage(rgb.jpg);
$image->setImageColorSpace(Imagick::COLORSPACE_CMYK);
$image->profileImage('icc', file_get_contents(/path/CoatedFOGRA27.icc'));
$image->negateImage(FALSE, imagick::COLOR_CYAN);
$image->negateImage(FALSE, imagick::COLOR_MAGENTA);
$image->negateImage(FALSE, imagick::COLOR_YELLOW);
$image->negateImage(FALSE, imagick::COLOR_BLACK);
$image->writeImage(cmyk.rgb);

This is how to Monochrome a jpg [on Windows].. since I couldn't find it anywhere else.

<?php
header("Content-type: image/jpeg");

$IMagick = new IMagick('c:\\testing\\fruit.jpg');
$IMagick->setImageColorSpace(Imagick::COLORSPACE_GRAY);

echo $IMagick;
?>

If your getting strange/bad color rendering from a PDF, after trying the colorspace constants noted by jdstraughan, try other values outside that range.

In one case for me only $image->setImageColorSpace(22) provided useful color. I have found posts elsewhere using values up to 255.

These are the basic steps to convert an image from RGB to CMYK:

$image = new Imagick();
$image->readImage(rgb.jpg);
$image->setImageColorSpace(Imagick::COLORSPACE_CMYK);
$image->profileImage('icc', file_get_contents(/path/CoatedFOGRA27.icc'));
$image->negateImage(FALSE, imagick::COLOR_CYAN);
$image->negateImage(FALSE, imagick::COLOR_MAGENTA);
$image->negateImage(FALSE, imagick::COLOR_YELLOW);
$image->negateImage(FALSE, imagick::COLOR_BLACK);
$image->writeImage(cmyk.rgb);

For more information: 
http://www.cesarebordi.it/imagemagick-php-convertire-rgb-to-cmyk-web-developer/