Roughly a third.
You can also typically discard high frequency data in your color channels. So not only are you not storing full precision data, you also don’t need to store roughly a third of the frequency bins. You can think of this like cropping the image but in the frequency domain. The data saving are identical to cropping the image but visually imperceptible.
You can see the sensitivity as part of the ppmtopgm program.
ppmtopgm (https://netpbm.sourceforge.net/doc/ppmtopgm.html)
ppmtopgm reads a PPM as input and produces a PGM as output. The output is a "black and white" rendering of the original image, as in a black and white photograph. The quantization formula ppmtopgm uses is y = .299 r + .587 g + .114 b.
This also has a spoof on Late Lament ( https://youtu.be/VNC54BKv3mc?t=109 ) that gives me a chuckle...
Cold-hearted orb that rules the night
Removes the colors from our sight
Red is gray, and yellow white
But we decide which is right
And which is a quantization error.