If I'm not mistaken, they adressed about this color problem in a live letter (I think... 2018 or maybe 2019...). So, they can't make a full spectrum slider for hair colors because there is a limitation to that on the game engine.
Like I said in some random posts here: we need a better engine to solve all our problems. From visuals to housing, to reporting system, we could definitely receive a revised and better engine capable of solving all the problems this game have. I know: they will
never do that. But a person can dream, right?
So, we have to use those washed up reds for our hairs in some hairstyles 'til sea swallows all u.u