I have had long hair, short hair (almost down to nothing) blue (and every other color) hair, a Mohawk (a real one, not a faux hawk) and dreads so I would be a hypocrite to not allow them to do that. It needs to be their own decision though and I would ask then to wait on the dreads and major color changes until 16 as the color is a lot of upkeep and they would need a job to pay for it and dreads are a bit of a commitment IMO so I would want them to be ready for it.
I am not fond of a mullet but whatever floats your boat ya know. It's just hair
Though we have the courage to raise our daughters more like our sons, we've rarely had the courage to raise our sons like our daughters. -Gloria Steinem