Because they are just a more mature stage of green bell peppers, yellow bell peppers are sweeter than the green ones.
3 tablespoons red wine vinegar
1 tablespoon water
1 1/2 teaspoons olive oil
3/4 teaspoon dried oregano
1/2 teaspoon Dijon mustard
1/8 teaspoon salt
1 garlic clove, minced
3/4 cup thinly sliced red onion
6 plum tomatoes, cut lengthwise into 1/8-inch-thick slices
2 yellow bell peppers, cut into 1/8-inch-thick rings
1/4 cup (1 ounce) crumbled feta cheese
Freshly ground black pepper
How to Make It
Combine first 7 ingredients in a small bowl, stirring well with a whisk.
Combine 1 tablespoon vinegar mixture and onion in a small bowl, tossing well.
Spoon onion mixture evenly onto a serving platter. Arrange tomato around edge of platter; place pepper rings in center of platter on top of onion. Drizzle remaining vinegar mixture over vegetables. Top with cheese; sprinkle with black pepper.