Purple is normally a colour that is associated with royalty, dignity nobility, as purple is the colour of the robe that royals wear. If we have a look at how purple is used in this poem, we can see how this symbolism operates. The poem begins with reference to the "little Gentian" who is laughed at by Summer because it tried "to be a Rose" and failed. However, as winter comes, this little gentian "ravishes" the hill and is described as a "Purple Creature" because there are so many flowers:
There rose a Purple Creature --
That ravished all the Hill --
And Summer hid her Forehead --
And Mockery -- was still --
The use of the colour purple in this poem therefore points towards the beauty and majesty of the gentian, in spite of its diminutive size and the way that Summer was said to mock it. The way that God has made it to flower in such vast quantities under cold conditions points towards God's overall plan in creation and also indicates that we should not try to be a "rose" if we are actually a "gentian." Each flower has its own place in the created order, just as we do, and everyone has the opportunity to "flower" and to show the world their dignity and nobility in the same way that the gentian does, under the correct conditions.