From the Super Mario Wiki
Silver Coins are coins that appear only when a P-Switch has been pressed, and they disappear once the P-Switch wears off. Although being called Silver Coins, they are more pale blue (in Super Mario World, both the pale Blue Coins and actual Silver Coins (pictured) appear).
Silver Coins first appear in Super Mario Bros. 3, where they appear in certain spots for as long as a P-Switch is in effect. Silver Coins have the same value as standard Gold Coins and are found usually in clusters.
In Super Mario World, pressing a Silver P-Switch turns all enemies into Silver Coins for a limited time. However, Silver Coins work differently from standard coins in this game; if enough are collected, they create extra lives, similar to jumping on numerous enemies in succession. In certain levels, a trick involving hordes of enemies and a Silver P-Switch can be executed to earn dozens of extra lives. This goes in order with collecting Silver coins:
In Diddy Kong Racing, silver coins can be found around certain tracks after beating a world boss and collecting several balloons. They are scattered throughout each track, sometimes hidden in hard to reach spots. Each one must be collected in all stages to complete the game. When taking the challenge, players must grab each coin while racing and win first place. If they manage to do so, they complete the track's challenge. In Diddy Kong Racing's DS Counterpart, Diddy Kong Racing DS, the Silver Coins were relatively unchanged except the absence of the Rare Logo and were no longer part of the story now having to be unlocked after getting at least bronze in the Touch Screen Balloon Popping Challenge.
Silver Coins appear in New Super Mario Bros., having the same role as in Super Mario Bros. 3. They sometimes appear in a trail behind Invincible Mario in a few levels, like World 7-3 and in a secret area in World 5-3. In subsequent New Super Mario Bros. games, the Silver Coins are replaced by Blue Coins.