Need a little help understanding the M163 G-code.

How does weight correspond to a color.

How do you calculate the P value for a color.
Using R, G, B how do you calculate P where
Saddle Brown uses R = 139, G = 69 and B = 19
So what would the definition look like?

M163 S0 P ??
M163 S1 P ??
M163 S2 P ??

thanks