Freckle Juice by Judy Blume

From the publisher:
Nicky has freckles -- they cover his face, his ears, and the whole back of his neck. Sitting behind him in class, Andrew once counted eighty-six of them, and that was just a start! If Andrew had freckles like Nicky, his mother would never know if his neck was dirty.
One day after school, Andrew works up enough courage to ask Nicky where he got his freckles. When know-it-all Sharon overhears, she offers Andrew her secret freckle juice recipe -- for fifty cents. It's a lot of money, but Andrew is desperate. At home he carefully mixes the strange combination of ingredients. Then the unexpected happens...