The Gown of Green is regarded as a badge of seduction;

Good morning, my fair creature, I'm proud to meet you here
With that sweet baby in your arms you seem to love so dear
Oh I think I know the father and you before I've seen
Don't you remember the day, my dear, you wore the gown of green

And from Thomas of Winesbury;

She's put off her gown of green
Stood straight upon the stone
And when she looked down her belly it was big
And her fair colour it was wan

From the internet;

In Portuguese we have a very old-fashioned expression: "casar na igreja verde" - "get married in the green church". The green church stands for the woods, the greens. So the meaning is to have an illicit relationship, actually out of a wedlock.