Digital BW, The Print

> which means
> that you have to multiply by 3 to get the number of colors
> possible

It's actually to the third power...as in, 8 bit RGB (8 bits per color) is
((2**8)**3) or 16,777,216, which is the same as 2**24th...

> (using RGB images) while DR refers the shades of grey
> between white and black.

Not necessarily gray, but shades (luminance)...there is a filter over the
sensor for RGB, or different colored lights are used.

> Assuming a constant DR for each color,
> they can be converted as demonstrated bellow.

Interestingly enough, the DR isn't that constant for each color.  It's
reasonably close, but can be off by almost a log factor of .2-.4 (as in 1.5
to 2.5 times the density difference) through the entire range.