Here is the item again:
You might think (A)
that a floor-length dress is
conservative (B)
until you see the one that Christina
wears. (C)
A psychedelic swirl of pinks and purples that attracts everyone's
attention.
You correctly realized that the period after wears creates a fragment. A psychedelic swirl of pinks and purples ... is an appositive, which cannot stand alone as a complete sentence.
To fix the problem, you chose to do this:
You might think that a floor-length dress is conservative until you see
the one that Christina wears. It is a psychedelic
swirl of pinks and purples that attracts everyone's attention.
Adding It is gives the fragment the necessary subject and verb to make it a main clause.
Good job!