Instructions
Proceed through the game until you reach the hotdog stand. Save the game when you arrive, so that if you get the wrong Yoshi color you can try again.
Grab the Yoshi egg off the top of the hotdog stand. It will follow you around. Your goal is to make it hatch in 18 to 19 minutes. If you wait too long or hatch it too quickly, it will be a different color. The egg hatches automatically when you run away from the Armored Harriers.
Start a timer, such as a stop watch or a cellphone timer.
Continue forward to the Glitz Pit. When you enter the pit, you'll square off against the Armored Harriers. Wait until 18 minutes have passed.
Enter the Glitz Pit and fight against the Armored Harriers. Immediately run away. You can't beat the Harriers. When you exit the fight, the Yoshi egg won't follow you. Instead, a baby Yoshi appears at your heels. If you timed it correctly, he'll be black.