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Ok, now take a better look at the values, the difference between white(255) and grey(128) is the intensity of light, so if the difference is the intensity if we illuminate with more intense light a grey object it will become white.
Let go to practice, look in a room with closed windows, you will see grey objects unless is very dark to see anything, now open all the windows and let the sun light enter, what was a grey object continue to be grey and what was white continue to be white even the illumination in the room increased one hundred times!
Some variable is missing!

Yes, your rods and cones.

A red object (or a white one or a blue one) appears gray in the dark not because of anything about them but because of something about you.  In low light humans have very poor colour vision, thus things appear to us as grayscale.  When you illuminate the room you increase the intensity of reflected light which provides sufficient photons to fire the colour-sensitive portions of the eye.