CMYK vs RGB in Photoshop.
The working difference with RGB vs CMYK modes in Photoshop.
Notice how the textures turn into flat colors in CMYK when they are adjusted with an overlay. The other thing that drove be crazy was towards the end of the video, turning plain texture to white and still having dark spots or turning it to black and having uneven blacks. It's not as clear in the compressed video but the CMYK file has blacks and dark blacks for some reason. I mainly wanted to show the basic differences with the modes and why it's just easier to use RGB and convert your flattened file to CMYK when you are finished. Always save your master file with all the layers in RGB though.
Hope this helps!