Red Onion vs Yellow Onion

Bottom line

Per 100 g, Onions (Red) provides 44 kcal, 0.9 g protein and 9.9 g carbohydrate; Onions (Yellow) provides 38 kcal, 0.8 g protein and 8.6 g carbohydrate.

That is a difference of 6 kcal per 100 g — small next to what the cooking method adds.

Yellow onion is the cooking onion: it turns sweet and disappears into a base. Red onion is sharper raw and holds its colour and crunch in salads and sandwiches, but goes a dull grey-purple when cooked for long. Use them where their behaviour fits, not by calories.

In short: yellow onion is the lighter option per 100 g, but with vegetables the choice is usually about texture and how they cook, not calories.