Silver and gold Yoshis shown in Super Mario World glitch demonstrations are unintended palette or sprite states. They are separate from the four normal Yoshi colors and are not required for any of the game’s 96 exits. There is no silver or gold Yoshi egg to unlock by completing the Special Zone.
The four normal Yoshi colors
Green Yoshi gains a shell power from the color of the shell it holds: a blue shell allows flight, a red shell becomes fireballs when spat out, and a yellow shell adds the ground-stomp effect. Blue Yoshi can fly with any shell, Red Yoshi spits any shell as fireballs, and Yellow Yoshi gains the ground-stomp effect while holding a shell. The shell is eventually swallowed, ending the held power.
For a useful early flight companion, enter Star World 2 and feed the opening star to the baby Blue Yoshi. It grows immediately. Finish the stage without losing it, then take it to a level with a shell. This provides controllable flight without a palette glitch.
Silver Yoshi in Outrageous
The silver or gray appearance associated with Outrageous comes from a sprite-handling glitch, rather than a secret character reward. A working setup depends on which enemies and effects have been loaded. Simply dropping Yoshi off the last ledge at an arbitrary Bullet Bill shot does not establish the same game state.
The demonstration below uses a specific early-level setup with Fire Mario, Yoshi, Wigglers and a Fire Snake before taking damage at the cannon. Reproduce its enemy positions and timing if you want to experiment, and keep an ordinary save separate from any suspend point used for attempts.
Silver Yoshi setup demonstration
Gold palettes and sprite glitches
A gold-looking palette is also an unintended state. Yoshi-related code can briefly continue after Yoshi disappears; another object loaded into the same sprite slot can then receive behavior intended for Yoshi. The result depends on the object and setup. A roulette item appearing as Yoshi falls is not a reliable universal recipe for a permanent gold Yoshi.
Use the normal Blue, Red or Yellow Yoshi routes for flight, fire and ground-stomp abilities. Palette glitches add no required collectible, and their appearance should not be mistaken for another completion objective.