Digital BW, The Print

I'll try again:

You have a machine that collects money.  You run it in your house and it
manages to find $10.49.  You also have a piggy bank that can accept only
quarters.  Based on this total from your machine, how many quarters do you
have for your piggy bank?  See the problem?

The machine is a color sensor or film.  The total amount is the RGB value.
The piggy bank is a different sensor, one that only accepts parts of the
spectrum.  It should be obvious that, since the machine (sensor) gives you
only a total (RGB value), and not the distribution of coins and bills (color
frequencies), there is no way for you to determine how the piggy bank
(another sensor or film with different spectral sensitivities) would fill
(or record light).

Thus, you cannot convert an RGB value in a way that precisely duplicates the
results from a different image-capture device.  Often, you can't even come
close.