Now, we're moving rather quickly... perhaps a bit too quickly for these old grey cells.

Quote - Well, I'm assuming that for generally pink skin, we know the value of k, such that:
r = kb

This bit... okay. The red component = blue * the ratio of red/blue

Quote - Let's replace r with kb in the first equation. The two equations are then
R = kb + s
B = b + s (true red value, little r, relates to the texture red value, big R)

Following you so far... in English:
"texture red value, big R" = the ratio of red/blue * blue + specular highlight.
"texture blue value, big B" = blue + specular highlight

Quote - Let's multiply the second by k.
kB = kb + ks

...so red/blue ratio * texture Blue value Blue = red/blue ratio * blue component + red/blue ratio * specular highlight.

Yes? If I've deciphered this correctly, then ... what does this do / why? This need clarifying before we go to:

Quote - And now subtract the first equation:
kB - R = kb + ks - kb - s

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks