You would say "an orange flower" or "an orange car," regardless of whether "flower" or "car" might be considered feminine in a language with grammatical gender.
However, if you're thinking creatively or conceptually, here are a few ideas of what might evoke a "feminine orange":
* Softness: Instead of a bright, bold orange, you might think of a peach, apricot, or coral.
* Delicacy: Perhaps a faded, pastel orange or a light, shimmering orange.
* Associations: Orange blossoms are often associated with weddings and femininity, so that scent or image could be considered a "feminine orange."
But grammatically, the adjective "orange" stays the same.