Where I live, spring and fall are short seasons, so I have in fact promised myself not to buy anything that will be worn in one of those seasons ONLY. A "spring item" or a "fall item" has to be wearable in at least one additional season.
That said, I enjoy wearing styles/colours/patterns/fabrics that somehow reflect or harmonize with the actual season. When it comes to sweaters, these are among the ones that I will happily wear in fall as well as in spring:
● thin turtlenecks in solid deep pink, red, blue, offwhite and medium grey
● thick offwhite turtleneck sweater
● thin floral turtleneck in wine + white + grey + black
● thin white turtleneck with black stripes
● thin cardigan in solid purple
● thick offwhite crewneck sweater with a sparse black pattern
● thin crewnecks in blue, light grey and medium grey
● thin stripey crewneck in grey + purple + offwhite
● long cardigans in offwhite, light grey, and icy white with black stripes