Also look at the diagram just below. As evident in the diagram, each primary color activates the layer containing the subtractive color opposite it . A given primary color does not directly activate the other two film layers. The red is common to each of these subtractive colors, with blue and green being filtered out. The same rationale applies to the other two combinations of subtractive colors.
White light exposes all three subtractive layers in the negative.
We must insert a fourth, special yellow filter layer below the yellow layer, because the dyes in the red and green sensitive layers below are also sensitive to blue, which this filter layer screens out and then dissolves away during developing. Since the sum of yellow and magenta is red, during development, the print film is red in the areas that are cyan in the negative. The same line of reasoning applies to the magenta and yellow areas on the negative, with green and blue resulting. We can tailor this statement for each of the other two negative colors. We can generate color transparencies by a similar color-reversal technique but without the need for a negative.