The additive principle only applies to mixing lights of different colours because the combination of two wavelengths creates the sensation of a third colour. This principle mixes various amounts of red, green and blue light to produce other colours. Combining two of these additive primary colours produces the additive secondary colours cyan, magenta and yellow. Combining all three primary colours produces white. 
The subtractive system creates colour by subtracting or absorbing certain colour wavelengths while reflecting others back to the viewer.