> One weird inconsistency I’ve noticed is that hair color is done via ZWJ, while skin tone is just modifier emoji with no joiner. Why? Seriously, I am asking you: why? I have no clue.

Mainly because skin tone modifiers [1] predate the ZWJ mechanism [2]. For hair colors there were two contending proposals [3] [4], one of which doesn't use ZWJ, and the ZWJ proposal was accepted because new modifiers (as opposed to ZWJ sequences) needed the architectural change [5].

[1] https://www.unicode.org/L2/L2014/14213-skin-tone-mod.pdf

[2] https://www.unicode.org/L2/L2015/15029r-zwj-emoji.pdf

[3] https://www.unicode.org/L2/L2017/17082-natural-hair-color.pd...

[4] https://www.unicode.org/L2/L2017/17193-hair-colour-proposal....

[5] https://www.unicode.org/L2/L2017/17283-response-hair.pdf