Cut each pound cake widthwise into seven 1-inch slices.

Trim a diagonal slice from the long sides of each cake slice, angling slightly, leaving a 1-inch width at one short end to create the xylophone shape.

Divide the vanilla frosting into four bowls.

Use yellow, green, orange, and red gel food coloring to tint each bowl of frosting a different color.

Carefully spread or pipe stripes of colored frosting onto each cake slice, alternating colors to mimic a xylophone.

Top the frosting with M&M's minis to add more color and decoration.

For the mallets, press a miniature marshmallow into one end of each pretzel stick.