New Post: cmyk image with only tones of black color

You could set the values to zero if you want to keep a CMYK image but I am not sure if that would create the same result. You can also set your image to Grayscale and then back to TrueColor right before you save it to keep a CMYK image as the output.

You are using Quantum.Max incorrectly, 0 = min and Quantum.Max = max for a pixel channel.