Rolling Egg Return
From the Super Mario Wiki
Rolling Egg Return is the Defensive Power Shot used by Yoshi in Mario Power Tennis. It involves Yoshi hopping inside a Yoshi Egg and rolling over the ball inside it. If players play the doubles tournament and complete the Mushroom Cup and the Flower Cup, they can power up the Rolling Egg Return, making Yoshi change into one of eight colors every time he uses it. The eight colors are red, yellow, green, light blue, blue, pink, black, and white.