One simple reason why this isn't happening, at least not any time soon: SIZE. It would require a reworking of literally every item in the game in order to implement this. There IS a mod that accomplishes this, but its current size including all packs is 18 gigabytes. However, it does appear to come in parts, so you can pick and choose what you want to install. It includes an option for hair only, for example. If you find the frankly crazy size of the mod acceptable, you can find it on Patreon: https://www.patreon.com/posts/color-sliders-1-46144228
