My return to Spriters resource (now VG resource)

[TheMario360]

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.