sorry i can't type chinese on this computer...
RGB mode is used in device that using LIGHT to show colour,
such as: monitor, projector, etc.
when 100%R 100%G and 100%B mixed together, it'll become WHITE
RGB is called ADDITIVE colour
http://www.colourtherapyhealing.com/colour/images/primary_colours_additive.gif
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CMYK mode is used in device that using INK to show colour
such as: printer, press, paint (watercolour and marker are good example)
when 100%C 100%M 100%Y mixed together, it'll beomce BLACK < but this is only in the theory
actually it's only a dark brown colour.
so BLACK (K) is added to get the REAL black colour, and also avoiding too thick of ink film on the paper (that will be another topic)
CMYK is called SUBTRACTIVE colour
http://www.steve.gb.com/images/science/subtractive_colours.png
For printing we're using CMYK is because the press is using INK to print but not LIGHT
of cause you would say I can use RGB INK to print, yes u can do that but the result is you'll only can print very few number of the colours out of RBG, however using CMYK u can print millions of colours. so u know the differences.
Hope it helps
u can email me if u have any other questions