A sleeveless cotton jacket.
It is especially worn by children and the elderly.
With no sleeves, it's easy to put on and take off, making it convenient for household chores.
There are various theories about the color, but they all share the commonality that the color becomes red when you accumulate experience points equivalent to 60 level-ups, which is the least common multiple of 12 and 10.
Lv.61: Red
Lv.66: Green
Lv.70, 77, 90: Purple
Lv.80, 88: Yellow
Lv.99: White
Lv.100: Pink