This may be something really stupid to say, but.....the services in the spa, including the hair stuff is WAY more expensive than having it done at home. I can't see why anyone would want to spend their cruise in a beauty salon.
I usually schedule something a day or 2 before we leave--mani, pedi, both, hair, or whatever. It gets me in the vacation mood and it is much cheaper than doing it on board. I did the massage on board last time--fun, relaxation. I love the rainforest. But no way I'm going to spend my cruise time getting my hair cut/colored/etc.