Gavinio is right on the colour regulations and Barca being the home team in the final, and Barca official website announced this statement on their tickets sales updates:
"Home team
FC Barcelona can also state that for the final against Manchester United, Pep Guardiola's
side will be seen as the home team and as a result will wear the familiar colours of blaugrana (their home kit) for
the match."
Therefore, the option is to wear our white kit which registered as an away kit at UEFA administration. But what confuse me is that with the blue short (which is part of the kit; im not sure if UEFA have the authority to asks us to change the designated colour so that we should wear a white short; although i know we used to wear white or black shorts with our home kit; and i never seen us wearing a different shorts with our away or third kits) the clash in colour still exists (Barca wearing blue shorts too), although not much visibly.
Another option as stated in the regulations, we as the
teams concerned should submit the case details to the UEFA administration in writing in due time. Given that Barca agreed, we can wear our third blue kit to celebrate the 40-year anniversary while Barca with their away yellow kit. Maybe our management already on talk with UEFA regarding this, that's why we yet to hear any official announcements.
Let us wait and see, but most probably i think we'll end up with the white kit.