This is a common problem that has a very easy fix.  This happens when the decal is placed on a flat surface.  The decals turn silver because light is reflected from under the decal. Light passes through the decal and is reflected back up from the picks and valleys in the paint.  Because flat paint is not smooth, it forms air gaps under the decal.


No matter what your final finish is going to be, if you are going to apply decals apply them on a gloss finish.  If you apply the decal to a gloss surface, it is smooth and allows the entire surface of the decal to contact the painted surface.   Since there are no gaps, light can not get under the decal and turn it silver.   After you apply the decal, give it a top coat of the desired finish. (Gloss, Semi-gloss or Flat)

There are two reasons that I know of that causes decals to turn yellow.  The first being that the decals are old.  Over time the carrier (the clear film that the ink is printed on) will get a yellow hue to it.  The second reason is that the manufacturer uses low quality decal material.  The higher the quality and thicker the carrier, the less likely the decal will yellow.

Most of the paint I use are made up of very fine pigments.  The Testors Model Master™ paints make up the bulk of my paints.  I drop a few BB's or stainless steal shot in each bottle.  This helps mix the paint.