The image of the knight who shows up (weapons and all) and says "thou must..." strikes a nasty note in a song about love and flowers. Enter the folk process:

There came a Knight, in yellow was he,
Fair are the flowers in the valley.
"My bride, my queen, I'll be loyal to thee!"
The Red, the Green and the Yellow.

A number of words might work where "loyal" is. True, faithful, loving, gentle...He offers her something rather than just wanting something for himself.

I've made a file of Sarah's tune, and I'm going to put it in my booklet of recorder tunes.