Cardigans are basic wardrobe pieces you can use year round. I love shopping Nordstrom Anniversary Sale to stock up on quality cardigans because I use them so much.
You can get a light material for spring/summer, and layer with a jacket for fall/winter.
For colors, having neutrals like grey and black is perfect to start with. Colors are fine, but you want to ensure you get colors that you use most of the time.