Hi!
I'm glad you are back. Last time I gave a short introduction and told you that digital cameras use three type of sensors to store the amount of incident light of the respective spectrum. Virtually any camera uses RGB (red, green, blue) sensors and stores the values individually. But this is not the only way to represent the color of a pixel. Printers for example utilize the CMYK color space because the three base colors cyan, magenta and yellow sum up to black if you where to print them on top of each other. They have an extra ink for black (the K is for black) to save colored ink, though. It is easy to switch between RGB and CMYK. All there is to be done is to multiply with a certain matrix. Simplest math.