Here is the original item:
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 wanted to do this:
You might think that a floor-length dress is conservative.
Until you see the one that Christina wears. A psychedelic swirl of
pinks and purples that attracts everyone's attention.
Adding a period after conservative makes a fragment. Until you see the one that Christina wears is a subordinate clause, which cannot stand alone as a complete sentence.
You might want to review the rules.