Look at how they're defined:
<GradientStop Color="#FCFC00" Offset="0.99999" />
<GradientStop Color="Silver" Offset="0.99999979" />
<GradientStop Color="Silver" Offset="1" />
This means that ... what, 0.001% of the mesh will be a gradient from FCFC00 to silver. That's not going to be detectable. Move the penultimate silver gradient stop to maybe 0.98 or something, and see if that gets you closer.