Key aspects of digital colour theory:
Print vs screen (additive vs subtractive)
light is reflected the more colours are layered on top of each other, by subtracting the colour it become darker and ends up with black.
Subtractive colour happens when you print a picture or mix paint.
The more colours your mix, the lighter it becomes. Additive colour is apparent in televisions, computer monitors and screen based images.
Cyan,magenta, yellow , key black.
CMYK-cyan,magenta, yellow, black.
GRB-red/green/blue- screen based
Duotone-two or more spot colours
Spot colour-one or more specifically mixed colours as apposed to a result of CMYK or RGB mix
Mono-like greyscale with a colour ie one colour plus stock