My return to Spriters resource (now VG resource)

[Hiynastrike]

The coins in Mario are usually a little more colorful than the ones you have here.
Maybe you could try a hue shift instead of shades as the colors currently look a little dull.

Before I post my sprites, I have one problem with your example. Your hue-shifting advice claims my sprites don't have that much contrast, but, they do.

What he means is your coin is kinda like a shiny plastic rather that a shiny metal...

Like this foil... It delves straight into the darker colors