Quote - ....In the RGB illumination model if something is grey or white depend on the illumination intensity.
In some way that I cannot explain, human eyes follow the physical model and not the RGB illumination model, and this has nothing to do with rods and cones.
If you know the incomming energy and measure the reflected energy, you are able to find the absortion coefficient and so, know how grey is an object.
Our eyes receive the reflected energy, so we know its intensity, but how our brain gets the missing parameter that is the incoming energy to complete the calculation???????

 

The missing parameter is called "White Balance". A lot of cameras can be adjusted for this but your brain does it automatically.

In your example, a piece of white paper in 50% illumination is reflecting the same amount of light as a 50% gray sheet in 100% light. Also the colour balance of the light can change and the paper would appear to change color.

The magic bit is that your brain knows that the paper is white and readjusts the illumination and colour balance so that the paper still looks white. A camera can do this using exposure controls and white balance (you tell the camera what is white and it adjusts all the other colours accordingly)