Cadmium Red Light, opaque

Opposite color, Thalo Blue Transparent.

To make a dark neutral with Cadmium Red light Opaque
mix it with the opposite hue Thalo Blue.

These words are colored in a tint of Cyan,
see how nicely opposites complement each other.
Cyan is called Phthalo Blue or Thalo Blue, depending on the brand.
Both words come from the chemical word Phthalocyanine.
The color's descriptive name is Transparent Cyan.

Why not use the color Cadmium Red Medium for the standard palette instead?

Cadmium Red Light is not as dark as Cadmium Red Medium.
Since it is easier to make Cadmium Red Medium or Deep by adding Transparent Magenta then to use Opaque Yellow in making a Red Light.
That I would consider a muddy chalk color.
You could do it by adding a Transparent Yellow which would tint up to a Red Light.
I found I use the hue Red Light more then Red Medium so the decision was made.