Rather than force you over there to read that post, I am replicating it right here, as it required a few edits for clarity.
“In order to understand the relationship of LAB, RGB, and CMYK, we have to understand that the colors are actually defined in terms of one another.
- Red is a primary in light.
- Cyan is the pigment opponent of red. It is defined as the pigment that reflects green and blue perfectly but no red at all. So in light, cyan is composed of equal parts green and blue, but no red.
- Green is a primary in light.
- Magenta is the pigment opponent of green. It is defined as the pigment that reflects red and blue equally, but no green at all. So in light, magenta is composed of equal parts red and blue, but no green at all.
- Blue is a primary in light.
- Yellow is the pigment opponent of blue. It is defined as the pigment that reflects red and green equally, but no blue at all. So in light, yellow is composed of equal parts red and green, but no blue at all.
Given these definitions, we can see why the pairs green, magenta and blue, yellow are called opponents. There can be no green at all where there is magenta, by definition. Shine a green light on a magenta surface and you see black; nothing is reflected. Magenta is defined in terms of what it doesn’t have, namely green.
[I edited the following part, as he was muddy in this section, contradicting later assertions – Terry]
To the extent that a color is created by combining magenta light [he is referring to the combination of blue light and red light] and green light, the color just gets grayer and lighter.
You can experiment with this in the RGB color space. To get yellow, set the red and green values to 255 and blue to 0. [You’ll see a very rich and bright yellow.] Now increase the blue. As you do so, the light does not actually become more blue, it becomes less yellow, paler and brighter. You cannot add enough blue to get a bluish yellow light. If you increase the blue all the way to 255, you get white light.”
[Note: In L*a*b mode, Lightness is expressed in the L channel, while in RGB lightness is expressed as separate values of red, green and blue — higher values meaning both brighter as well as more saturated color in RGB.]
I love the thought of there being a bluish-yellow light, as he suggests, or a greenish-magenta, or a cyanish-red! But then again, Alice Through the Looking Glass is one of my favorite books as well!! Such colors are impossibilities according to the definitions above.
For Further Reading:
See previous post, “Some Color Theory and Photoshop LAB mode – Warm-up for Full Article“