The book was about a fairy who meets two(?) human girls and befriends them and they go on little adventures together. I believe it was a series with every book having a new color based fairy. A red fairy, a green fairy. It also had a Christmas fairy in one.
Lost Classics of Teen Lit: Name That Book! 
this sounds like Scholastic’s Rainbow Fairies books. There were a ton of offshoot series, and more than one Christmas one.
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