ah, now i understand your problem :D should've read your first post more carefully. i would use the texture sample only for the opacity and do the color of the decal with a constant. that way you can avoid getting color bleeding due to mipmaps and the dxt compression. inverting inside the material editor works via the…
he inverted his diffuse because they are b/w and can be used as a opacity map for the translucency. if your decals aren't b/w that obviously won't help you a lot. depending on what the decal look like you could extract a mask from the diffuse by clamping values, but you are probably better off just using an alpha... so…