Choose Yoshi's Color (Practice Mode)
In Trial Mode, pick the Yoshi of the color you want and press Start and Z simultaneously. Press B and go the practice mode.

Different Colored Yoshi's
Black #1
On level 2, page 4, go to the room with the vines coming from the ceiling. Hop from one vine to the next quickly until you get to the floating bubble. Burst the bubble and grab the egg. After defeating the level, you'll find a black Yoshi on the next page.

Black #2
On the second level, second page, head to the tulip to the left of Miss Warp, eat it, shoot upwards, grab the black egg near the ceiling, and defeat the level like you normally would.

Black #3
On level 2-1, go past the tulip a little, eat it, bounce up to a hole, hit the ? ball with an egg, and grab the black egg.

Black # 4
On level 2-3, go through the stage until you find a help box that tells you about a big egg. Then, go left and jump on the last floating rock, then the one above that. Shoot the ? with an egg, get the black egg, and beat the level.
(Thanks to jera@cutey.com)

Purple Yoshi
Collect all the coins in any level.
(Thanks to LWargalla@t-online.de)

White #1
On page 3 of the third level, go down the pots until you get to the dog. Head down the red pipe, grab the white egg, and defeat the level as usual.

White #2
3-2: Keep going up until you see a pot. Go in the pot, come out, and go over until you see a feather in the air. Don't jump on the feather, but go down the stairs. Shoot the bubble with a ? that you come to, and grab the white egg.
(Thanks to Cyberworm4@aol.com)

Infinite Lives
For infinite lives, reset the game if you die. You'll resume play on the level you died with all Yoshi's you had when you started that level. However, you won't be restored to your last checkpoint.
(Thanks to Crispped87@aol.com)

Mario Paint Tune
At the Trial Mode menu, wait for a few minutes, and you'll get a hidden tune.
(Thanks to SCUZ21256@aol.com, jera@cutey.com)

Pop Bubbles Without Eggs
Keep bumping/jumping on it.
(Thanks to jera@cutey.com)

Resurrect Yoshi
Get a white shy guy and complete the level. Head to the character select screen after the level and select the shy guy.

Rotate Pop-Up Pages
Press the C buttons to turn the pop-up displays and press R and Z to zoom.
(Thanks to Monkeybutl@aol.com)

Save Black & White Yoshi
Beat the game with black and white Yoshi's. When you start a new game in story mode, you'll have the new Yoshi's to select from.
(Thanks to jera@cutey.com)

Secret Message From Yoshi
At the end of each mini game, get enough for a heart fruit and eat it. After all the letters appear, arrange them properly, and you'll get a message from Yoshi.

Small Eggs
In 4-4, get shrunk. Then, hit an egg block.
(Thanks to jera@cutey.com)

Walk Under Lava
In 2-2, go until you reach the first heart and jump on Blarg's head, get the heart, and head back to Blarg.
(Thanks to jera@cutey.com)

White And Black Yoshi's In Ending
Beat the game with black/white Yoshi's.

White ShyGuy Locations
In 1-1, go left of the second heart. The ShyGuy's in the bubble.
In 1-2, go right of the third Miss Warp. The ShyGuy's in the mystery crate.
In 1-3, go right from the melon delivery The ShyGuy's in a bubble.
In 1-4, go past the umbrella. Go down until you come upon 5 blocks. Shoot the one farthest to the left.
In 2-1, go down the first vase. There should be a wall made out of blue rocks which you should blow out. Go through and then you come to blue rocks on the ground. The one on the bottom holds the white ShyGuy.
In 3-3, go through the level until you come to a fork in the path and each path is leading to a different vase. Go down the path that leads to the barking dog. Go down that vase and you should come out on a path that has two box's with question mark's on them. Ground pound on the first box and a white ShyGuy will appear.
In level 4-1, Jungle Hut, look for four blue blocks. Hit the upper left block with an egg. A white ShyGuy will appear.
NOTE: These tips only work if one of your Yoshi's is dead.
(Thanks to chinadoll_faerie@hotmail.com)