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This is really cute. The joke is more pronounced in the manga. There's a two panel/page joke of Azusa being fully tan (after the club goes with Sawako to the music event) and then the next day she's back to being untanned.
Also, the shading of Azusa looked fine on my phone but not so much on my monitor.. if you can't see it easily.. she basically gets a full tan before Ui can put on the sunscreen.