I think I'd agree, Malcolm. I can't see a connection between the yellow and blue handkerchiefs. Sam Henry's Songs of the People has two versions. Here they are:

THE PRETTY BLUE HANDKERCHIEF (I)

As I went a-walking one morning in May,
I met a wee lassie come tripping this way,
Her cheeks were like roses, so sweetly she sang
With her bonnie blue kerchief tied under her chin.

Said I, 'My wee darling, what brought you this way?'
'I'm going to my work, kind sir,' she did say,
'I'm going down to yon cottage my wool for to spin,
With my bonnie blue kerchief tied under my chin.

'Why do you wear it all on the one side?'
'It's my country fashion, kind sir,' she replied,
'The fashion you know I should like to be in
With my bonnie blue kerchief tied under my chin.

This couple got married, as I hear them say,
The drums they do beat and the music doth play.
She's no more the poor spinner, but wears a gold ring
And he the blue handkerchief under his chin.

tune: Master McGrath (sounds like "Sweet Betsy from Pike" to me)

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Click to play

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THE PRETTY BLUE HANDKERCHIEF (2)

As I was a-walking one morning in May,
A bonny young lass came a-tripping that way;
With cheeks red as roses, she cheerfully did sing,
And a bonny blue handkerchief tied under her chin.

'Where so quickly?' I asked her, and caught her slim waist;
'To my work, sir, I'm walking all in a great haste:
To work in yon factory where cotton they spin,'
With a bonny blue handkerchief tied under her chin.

'Why wear you the kerchief that covers your head?'
'It's the fashion they keep in the country,' she said,
'And you know that the fashion we like to be in,
With a bonny blue handkerchief tied under my chin.'

Why wear you the colour that glows in your veil?'
'Because it's true blue, sir, that never shall fail,
Like a sailor's blue jacket that fights for the king,
With a bonny blue handkerchief tied under my chin.'

When I found her so charming, how could I forbear?
I flew to her side, I called her my dear,
'My own dearest jewel, come take the gold ring,
With a bonny blue handkerchief tied under your chin.

'To church we will go and be married with speed,
A loving young couple and happy indeed;
When the day's work is over so cheerfully you'll sing
With a bonny blue kerchief tied under you chin.'

tune: Master McGrath