From the Super Mario Wiki
Each Yoshi has a Favorite Fruit in Yoshi's Story, which revolves around eating fruit to advance through the game. Each color of Yoshi has a different Favorite Fruit, which makes them happier. If a Yoshi eats its Favorite Fruit, three times more ♥s and health than normal. Additionally, while each color of Yoshi has a unique Favorite Fruit, all Yoshis love the Melon, which also awards a bonus 100 ♥s apiece after the stage is completed.
The Lucky Fruit, which is chosen at random before each playthrough, is preferred over the Favorite Fruit, awarding eight ♥s and completely recovering the Smile Meter.