The precise boundaries of a HOH relationship obviously have to be negotiated between the people involved. Some might include Tattoos and hair colouring in their list of "restricted practices" while for others they might not.
In our house, a tattoo would best be negotiated. K has no objection to me getting one if I want one - she has one and I don't so it would be a bit churlish of her to refuse to let me have one! On the other hand, I have a lot of respect for her opinions and would certainly discuss and negotiate with her before getting one. But that would really be done in her role as "my best friend" rather than as HOH.
If I went off and got a tattoo without consulting her, she'd probably feel a bit hurt that we had missed the chance to do it together. But I don't think it would be a matter for discipline in our house. Unless I decided to get "Satan Spawn" tattooed across my forehead!!! But certainly a discreet tattoo would be OK.
Same for purple hair I think.
But that's just us.
Now if I went out and blew the grocery budget on a new pair of shoes, that would be a VERY different matter. I'd be a dead duck!
I know this because I almost bought a £200 handbag a few weeks ago. I managed to resist and was telling her about it. It was made very clear that handbags cost no more than £50 - EVER. And that, as they say, is the END of the story. I almost argued but I know better than to argue with that tone.
But that's our arrangement. For others the accessories budget may be huge but tattoos and purple hair off limits. It's very individual.