On a plant:
Bell peppers come in lots of colors: red, yellow, orange, even purple when fully ripe. Most green peppers are just unripe peppers!
Onion bulbs grow underground. Above ground, onion leaves are tall and thin. For each leaf, a ring of the bulb grows.
On a tree:
Olives are small, round fruits that grow on trees, like cherries! But unlike cherries, olives are naturally bitter, not sweet.