Silvering is caused by the decal not adhering to a totally smooth surface.  While a flat-sheened or metallic surface may appear smooth, at a microscopic level it's actually very rough.  The decal isn't soft enough, even with setting solution, to conform to all the microscopic pits in the surface.  Light passing through the clear part of the decal film bounces off the model surface and reflects back up through the film - silvering.

You need to give the model, or at least the spots the decals will go, a coat of clear gloss.  Gloss has little to none of the surface defects that flat and metallic have so when you lay down the decal, the clear film around the edge should disappear.  Then just hit the decal with clear flat coat.