What we see as white light is actually made up of light of different colors. The figure to the right represents the visible spectrum.

Coca Cola is not "clear" but has a distinctive color to it. Using a scanning spectrophotometer we can "see" in a more exact way what color Coke is. Coke absorbs more light in the blue end of the spectrum than in the green and red areas.

This is not a perfect filter, letting some light through in the "Red" region, but it will work for our purposes.