Colors Part II

Last post, we looked at the primary and secondary additive colors. Today, we will consider the primary and secondary subtractive colors. Recall that subtractive colors basically describe pigments, that is, substances that reflect certain colors of light and absorb others. Let us reconsider the pigment chlorophyll, which was used as an illustration of a pigment last post. When white light shines on it, it reflects green light. White light, however, also contains red and blue, since white light contains all three primary additive colors. What happens to these colors? They are absorbed by the pigment. To get a little technical, the energy from the absorbed light becomes energy in the chlorophyll molecule, but since we are focusing on light, we need not concern ourselves with what happens to the absorbed light, all we need to know is that whatever light is reflected by the pigment, the other primary additive colors are absorbed (they “disappear”).

What are the primary subtractive colors? Much like the primary additive colors, these will be the colors that can produce any other color when the pigments are mixed. The primary subtractive colors are yellow, magenta, and cyan. To understand why this is the case, consider a yellow pigment. What happens when white light is shone on yellow pigment? Yellow light is reflected. Since yellow light contains red and green, we can think of yellow pigment as reflecting red and green and absorbing blue. Similarly, magenta pigment reflects red and blue and absorbs red and cyan pigment reflects blue and green and absorbs red.

What happens when two primary subtractive pigments are mixed together? Let us consider mixing yellow and magenta. Yellow reflects red and green light but absorbs blue while magenta reflects red and blue but absorbs green. Thus, the green reflected by the yellow pigment gets absorbed by the magenta pigment while the blue reflected by the magenta pigment gets absorbed by the yellow pigment. If we remove green and blue, what is the remaining reflected color? Red. Thus, red is a subtractive mixture of yellow and magenta. In a similar fashion, yellow and cyan pigments produce green and magenta and cyan pigments produce blue. Thus, red, blue, and green are the secondary subtractive colors.

What happens when the three primary subtractive colors are all mixed together? Well, yellow pigment absorbs blue light, magenta absorbs green, and cyan absorbs red. If all three pigments are mixed together, they all absorb each other’s reflected light, so nothing gets reflected. Since no light is reflected, the resulting pigment is dark, that is, black. Thus, black pigments absorbs all colors of light and reflects none. Notice that complimentary colors also exist here. Since yellow light reflects red and green and absorbs blue and blue light reflects blue and absorbs red and green, yellow and blue pigments together absorb all three primary additive colors and reflects none, producing black. Thus, yellow and blue are complimentary colors. In a similar fashion, magenta and green are complimentary and cyan and red are complimentary.

The preceding discussions of primary and secondary additive and subtractive colors may seem strange. After all, the colors of the rainbow are right there and using red, yellow, and blue as primary colors just makes so much sense. However, note that, no matter how much we have been taught that red, yellow, and blue are the primary colors, the primary and secondary additive and subtractive colors is the way things work. For example, consider the following picture, which shows the progression of printing color images. The first row shows the primary subtractive color yellow bt itself, the second adds magenta, and the final adds cyan. Notice that every color can be produced by using these three pigments.

Additionally, consider the color selection in programs such as Paint or Photoshop. Each color selected is given as a combination of three colors: red, green, and blue. In fact, you can often put in numbers to change the amount of each color in order to change the selected color. These programs are mimiking primary additive colors for their color selection. Again, it may seem strange, but that is just the way things work.

That sums up subtractive colors. Next post, we will tackle the thousand dollar question: if red, blue, and green are the primary additive colors and yellow, magenta, and cyan are the primary subtractive colors, then what is special about red, yellow, and blue?