Unwins Aster Dwarf Queen Mix Seeds or China Aster are a half-hardy annual that's a compact, bushy plant that's free flowering. You'll get a wide range of colours with Dwarf Queen Mix in purples, pinks, reds, yellows and whites.
It's a very reliable performer that's just perfect for pots. Aster Dwarf Queen is also late season flowering so you'll have plenty of colours through those autumn months.