Black Yoshi(2-4)
Go to the #4 Torrential Maze on Page 2. When you start, keep heading
upward through several rooms until you get to a room that has ghost
vines coming from the ceiling that you can barely see from the ground. Jump from vine to vine, if you're fast enough, you'll come to an edge with a floating bubble. Pop the bubble and grab the large egg that looks like a soccer-ball. Finish the level as usual by collecting fruit. When you turn to the next page, a black yoshi will be with the other six. This Yoshi likes to eat everything, and he gets three hearts for eating any fruit or shy-guy. His eggs also make a larger explosion than the regular ones.
White Yoshi(3-3)
On Page 3 Area 3 starting from Miss Warp 3 go right. When the road
splits with a vase at each end, go in the upper vase. Then go right and enter the upper vase. You should see a dog and a red/orange pipe south of you. Go right and go into the first red/orange pipe to enter a hidden cave. Go right and throw an egg at the bubble. You will now receive a large white egg. Eat the remaining fruit on the level and on the next page and you will be able to choose the White Yoshi.
Credit Goes To: IGN 64