When you need to filling black color in designing, if the color is C=0、M=0、Y=0、K=100, is monochrome black.

If it is C=100、M=100、Y=100、K=100, is 4 color black( Not say that it must be 100%, as long the black color combine with 4 color, we called 4 color black.

When we use the designing software, if no need to consider the printing when filling the black color, we will stretch the darkest color on the software directly from the color picker. (If need gray color, just pick some gray color)

We can use CMYK combine black color, and also can use singal black ink color print the black color.

If use 4 color ink combine black color then printing, it will have difference when we use printing machine, especially for small text, there may be inaccurate overprinting.

4 color black color will not black like monochrome black.The gray is also the same, because it will become more green, more red or the color is uneven.