As the new term approaches, there will probably be staff at the school/sixth form centre during the week. Why not give them a call on Monday to explain the situation and to see if there will be a problem.
6th form policies vary enormously. In dedicated sixth form colleges (which aren't part of a school) the policies are usually close those of universities. (i.e. anything is acceptable as long as it's not offensive or could otherwise annoy fellow students). In schools where there are very rigid rules applied to pre-6th form students the policies might be much stricter (because they don't want the sixth formers to 'set a bad example' to younger people).
In the (non-uniform) school I taught at, blue hair would be no problem at all. Indeed, I seem to remember that a cross-dressing male sixth-former (who must have spent a fortune on his dresses and make-up) actually went through a blue hair phase at one time!