I think its his favorite colour, why? well most of his items are yellow lets see: Beserker's greaves, Static shiv, Trinity Force, Wiggle's Lantern and Executioner's calling (at least i do it just to piss off the enemy and its good against carries whit lifesteal).

It may sound silly but think about it its very curious that some of the best items for Shaco are yellow and most of the time when you see a Shaco player they usually have Static, greaves and wiggles, like 90% of the time.

Just think about it