True Summer gets almost no yellows; the yellows it does get are pale and dusty but also totally cool -- like lemon chalk.
True Summer yellows are elusive in fabrics, so a typical True Summer almost never finds a yellow that looks good on her and thinks it's one of her worst colors.
Light Summer, though, can handle a range of cheerful, buttery yellows. On a Light Summer, yellow often picks up yellow in the hair or in the eye.
This difference is part of the reason why Light Summers are often convincing blondes but True Summers seldom are. Blonde hair is essentially a big swatch of yellow hovering around one's face, you know?
If you're not sure of your season, try at-home draping.