。
She has a dark complexion.
Literal
She [topic-は] color [subject-が] dark.
色が黒い (いろがくろい) is the counterpart to 色白: 'having dark/tanned skin.' The が marks 色 as the subject of 黒い. While 色白 is a na-adjective used as a single unit, 色が黒い keeps its analytical structure as a [noun]が[adjective] predication.