File mode: CMYK CMYK working profile: Fogra 39 Grayscale working profile: Dot gain 15%
Any region with pure black (0 0 0 100), if I turn off the cyan, magenta and yellow channels, it gets slightly darker, almost to rich black. Why is this?
When Photoshop shows you a single channel, it does so in a greyscale, rather than the actual colour. This greyscale is mapped to RGB greys, and thus 100% black is displayed as #000000. Which is a darker colour than CMYK 0/0/0/100, especially if you've set Photoshop to display CMYK blacks accurately.