What happens if I put elder gray colors in the custom bin? Will my sims be able to age up with the same hair mesh they have or will I have to edit them to use the elder hair via mirror? Reason why I'm asking is that I'm considering using multiple hair colors but just 1 gray color for all of them.
^ The gray needs to have the same "Family" number as the hair colors you want to turn into that particular gray. Don't put more than one of each of the binned colors in the same family, as there's a small chance they'll end up "fighting" each other, especially if an age is missing or you age down sims from elder ingame/in CAS/Bodyshop - I've had a few sims that grew up with the wrong hair color because of this (the game chose the wrong blond/brown/etc. because some hairs were familied together).
You can have a separate gray in the custom bin that works for either all ages or just elder. Cat's hair binner allows for this. Nice if you want an option with colored hair for elders (you'll have to apply the gray hairs in a mirror, custom bin) or want separate files for elders but familied to the binned hairs (will self-apply but if you want a different grey color you'll have to use a mirror).