Having trouble making this decision? These two seasons are often confused -- probably because people in both seasons are flattered by warmer colors and can take a lot of saturation.
There are a few colors that will distinguish them, though.
If you're a Dark Autumn, you will look good in some light golden browns, while Bright Spring can't do any kind of brown. As a Dark Autumn, you'll also look good in dark rust red, while Bright Spring doesn't have anything close to that.
If you're a Bright Spring, you can wear a whole value range of fuchsias, from pretty deep to very light. (Dark Autumn has some coolish reds, but they're very dark -- more like wine.) As a Bright Spring, you can also wear a light, sunshiney yellow, while Dark Autumn's yellows are more like gold.
Still can't figure out your season? Try at-home draping.