When Elvis Costello takes Homer's guitar and drives back to get the pick, the guitar changes color from red to a mustard sort of color.
When the Rolling Stones are on the stage talking about the camp, they knew Otto's name even though they never met him before.
When everyone is getting told that they can't all play guitar and are told that someone could play bass instead, Homer jumps over the counter and gives guitars to everyone, including the bass nobody wanted before.
After Homer and his gang get their instruments (all guitars)... and started singing, Homer's guitar changes form and color.
The Gracie Sound is Mick Jagger shouting "Simpsoooon!"
As the end credits roll, behind the scenes of the rock stars recording their voices are shown.