Theory:

“But I can get a hair-dye
And set such colour there,
Brown, or black, or carrot,
That young men in despair
May love me for myself alone
And not my yellow hair
 
“I heard an old religious man
But yesternight declare
That he had found a text to prove
That only God, my dear,
Could love you for yourself alone
And not your yellow hair.”
Explanation:
 
Yeats discusses the concept of beauty and how the world perceives it in the poem. People look at external beauty and judge their personalities. The poet says that Anne Gregory is liked by every man only because of her beautiful yellow hair and not for who she is as a person. We all like to be loved for who we are as a person, our character, our likes, our beliefs, our principles etc.
 
Anne Gregory is slightly taken aback by the poet's words that any man who has despaired for her love has done so only by seeing her beauty. Facial features and hair colour are temporary and keep changing as a person ages. For example, one might have blonde hair; when they age, the density could be reduced, and the person could get grey hair. So Anne Gregory knows it is dangerous for people to fall in love with her because of her hair colour and beauty. Whereas qualities like kindness, helping others etc., are permanent, and these are things that make a person who they are. Anne Gregory, therefore, suggests an extreme step to avoid falling into the trap of being loved for her beauty alone. She says she would colour her hair with hair dye, making her less attractive. She would then know that all men pursuing her are only there for her, not her beauty. She suggests using hair colours like black, brown and even carrot colours, which might look less impressive on her hair. She hopes that men might love her for her personality alone if she does so.
 
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Hair colours
 
The poet tells Anne Gregory that it is no use to colour her hair differently, as men might still look for her other attractive features. He also says that he heard an old religious man declare that it is not possible for any human being to love someone for their inner beauty. He bases it on a religious text as well to support his statement. He says that only God could love her for who she is as a person and not for her yellow hair. The poem thus focuses on the fact that God's love is the only unconditional love for a person.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1
DyeA natural or synthetic substance used to add colour
2
CarrotA bright orange colour
3
YesternightYesterday night
4
UnconditionalLoving with no conditions
Reference:
National Council of Educational Research and Training (2006). Beehive. For Anne Gregory - William Butler Yeats (pp. 140-141). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.