The Collaborative International Dictionary of English v.0.48:

Amaranthine \Am`a*ran"thine\, a.
   1. Of or pertaining to amaranth. "Amaranthine bowers."
      --Pope.
      [1913 Webster]

   2. Unfading, as the poetic amaranth; undying.
      [1913 Webster]

            They only amaranthine flower on earth
            Is virtue.                            --Cowper.
      [1913 Webster]

   3. Of a purplish color. --Buchanan.
      [1913 Webster] Amaranthus

WordNet (r) 3.0 (2006):

amaranthine
    adj 1: of or related to the amaranth plant
    2: of an imaginary flower that never fades [syn: amaranthine,
       unfading]