So how did the word amaranth go from being an imaginary flower to a genus of flowers by the likes are the deep red cockscomb, love-lies-bleeding, and prince's feather; to A color inclining to purple as Webby 1913 says? The entry for amaranth in Random House Webster's College Dictionary goes into more details about the story behind this interesting word:
    English amaranth, which first appeared in the 16th century, came from Latin amarantus, meaning not just 'flower', but 'an unfading flower'. The Greek word from which Latin amarantus derived is amarantos which is formed from the prefix a- meaning 'not' (called the "alpha privative") plus marainein meaning 'to wither or decay'. The Indo-European root of this word, mer- meaning 'to die', is also the source of such words as "mortal, murder," and "mortgage."
Anthos is Greek for flower as in polyanthus, a kind of primrose or narcissus that has "many flowers," so -ant was reshaped to read -anth even though it had no true connection. The Roman naturalist Pliny first wrote in the first century A.D about his imaginary amaranth and said it never faded. Clement of Alexandria said a hundred year later that the flower was a symbol of immortality. Whether he knew about the Greek amaranth isn't known although he may not have simply transplanting an actual earthly flower to heaven when he spoke of a crown made of amaranths. The etymology "not-fading" and the reference in 1 Peter 5:4 to an "unfading crown of glory" led Clement to invent his flower which, true to its name, never fades. Some genus of amaranth are used in diets as a source of protein. They are annuals, tall with seed heads that droop. The large flowers and foliage is usually showy and a bright gold and purple. The grains are used in cereals and they range in a wide variety of sizes and typically white. In a suburb of Brisbane a Greek gardener says that one of her wedding presents was a packet of family heirloom amaranth seeds. Perhaps so that their love would never fade.

An undying flower of rare color from ancient legend. What poet could ask for more? By 1667 Milton was talking about the ever blooming plant his epic poem Paradise Lost:

    Immortal amarant, a flower which once
    In Paradise fast by the tree of life
    Began to bloom; but soon, for man's offence,
    To heaven removed, where first it grew.
William Cowper describes the amaranth in his poem Hope as pleasures exempt from oblivion when he wrote in 1781:
    "Hope plucks amaranthine joys from bowers of bliss."
In one of his poems written 1858 about a couple of angels Henry Wadsworth Longfellow describes one of them as:
    "The angel with the amaranthine wreath, Pausing, descended."
Many thanks to Gritchka for explaining to me about the the Greek origins! So there you go, that's how the Romans went from from 'unfading' to 'flower' with the word amaranth.

Sources:

The Maven's Word of the Day:
http://www.randomhouse.com/

Seed Savers Handbook:
www.genevar.com.au/seedsavers/handbook

Public Domain text of the poem taken from E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.” Am’aranth.”:
http://www.bartleby.com/81/597.html

Wreaths:
web.odu.edu/webroot/instr/sci/plant.nsf/pages/wreaths