Hi, to start it is needed to read my last post to fully understand this one.

Today I will say more about color mix to be sure to understand the following.

All I presented on my previous post was about the emitting of colored light. Whether for direct light as the sun which emits white light :

or diffused and reflected light rays.

Thanks to the Venn diagram we saw that if we add blue, red and green lights we obtain a white light. This is what we call the Additive synthesis because we add different lights' color.

What we gonna attack now is the Subtractive Synthesis.

To begin, when we see an object's color, it isn't making the light our eyes receive. We can see it because some light is reflecting on it and then the light ray finds his way to our retina. But why we can see a red table and a green chair if they are lightened by the same light ( I'm aware that a red table and green chairs isn't the best combo to have a tasteful decoration PS : I am sorry if one of you have a red table with green chairs, don't hesitate to share a picture to prove me I'm wrong )

An object owes its color to its wavelengths absorbtion.

Imagining that the table and chair are of pure color ( only one color wavelengths ). That means that they absorb all others wavelengths submitted to them and only reflect the color we see. We take the previous Venn diagram in an other way :

  Here primary colors are cyan, yellow and magenta.

The red table reflects the red and absorbs the cyan, the blue and green, while the chair absorbs magenta, blue and red.
So far everything should be still simple. The following begins to be a little more complicated

Let's keep the red table and green chair exemple and let's add a magenta cushion. ( I'm realizing that I have the same kind of exemples as the teachers in high school. ). We already saw what it is like under white light, now let's try with colored ones.

• Under a yellow light (mix of red and green) :
  1. The red table absorbs green wavelenghts and reflects red ones. It appears RED
  2. The green chair absorbs the red and reflects the GREEN.
  3. The magenta cushion absorbs the green and reflects the red. There is no blue to reflect so it appears RED.
• Under cyan light (mix of blue and green) :
  1. The red table absorbs the green and the blue. It reflects nothing so it appears BLACK 
  2. The green chair absorbs blue and appears green.
  3. The magenta cushion absorbs the green and reflects the blue. It appears BLUE.
•  Under magenta light (mix of blue and red) :
  1. The red table absorbs the blue and reflects the RED.
  2. The green chair absorbs red and blue, so it appears BLACK.
  3. The magenta cushion reflects the blue and red and doesn't absorb light. It appears MAGENTA.
  This part will really be important for the following posts.

We can understand that a black object under white light is an object which absorbs every color's wavelenghts and a white object is an object which reflects all wavelenghts. That is why a black object heats more under the sun than a white one, because it absorbs all the photons energy while the white reflects it.

The Subtractive Synthesis occurs also when filtering the light. To take an example if we put a filter in front of a light, it will only let pass its own color through.

In this case, the yellow filter lets the green and red passing through it. In the case of a blue filter, it would only let the blue pass.

Understanding those two concept which are the Additive Synthesis and the Subtractive Synthesis will be essential to be sure tu understand the following.