Think of it as a negative process the result of which, the output, will be the opposite of the input.
Originally Posted by dhroane
As in black and white, the print gets darker where the negative is lighter, the print will get more cyan where there is less of it in the negative.
So a print that is too cyan needs more cyan to become less cyan.
Just like a black and white print that is too light needs more light to become less light.
When going from positive to positive (scene to print, or slide to print), the opposite is true, and you simply need to remove what there is too much of.
So if the print is too cyan, you need to use a 'minus cyan' filter in front of the camera lens to get a correct print without having to use filters to correct for it at the printing stage.