This is a good idea! I would love it if your Sim could work as a hairdresser as well. It is a fairly common type of work that really should be in the game. And I always love more hairstyles. I buy any pack if it has new hairstyles!
I would also like it if all haircolours and styles could add grey hairs for your aging Sim.
And if your Sim colours its hair, the hair could grow and start to show your real haircolour and then need to colour it again or let it grow out. We already have body- and facial hair that grows.