I wonder, when applying a linear transformation like in the shader described, will the total available color space decrease? Simply put, if a one-dimensional color value on the arbitrary scale between 1 and 100 needs to be decreased by 20 for correction, the resulting maximum will be 80. Does that mean the total available color values will be less?
This can easily be verified with a simple thought experiment: imagine an area is almost completely red. This area will have to be complemented with full blast of green and blue to even achieve white, or partial blast of green and blue to achieve gray.
It can not achieve any color without a red component, hence reducing the area of the color triangle for that part of the screen.
Basically, what you are doing is adding a cast to the image. This cast cancels out with the cast that the backlights give. When you add two complementary color casts to each other, you end up with neutral gray.
This results in an image which is darker, but still has the full color range that the TV is designed for.