Paying homage to K.A. Tsokos
You can fact check this on Quora but human hair and the root of the Purple Hair Model is governed by the Conservation of Purply. Hair is assigned a Purple Number (PN) = 1 and all other variations of hair color are simply 1/2 integer values of PN. Brown hair = purple + anti blue (yellow).
1/2 = +1 -1/2
LOL this one gave me a good laugh