Not sure - lots of dresses are coloured to give the rhyme for the next line - e.g. Cocaine Blues has loads of them... John Hardy in different variants has a girl dressed in red who comes to his hanging ground and says Johnny I've come to see you dead / a girl dressed in blue who says Johnny I'll be true to you.
Of course Purple means you ar an Emperor, Scarlet means you are a Cardinal and a White means you are the Pope... Oh, and a little balck dress means you are Mary Quant (or was it CoCo Chanel?).