Decals can are usually printed on one of 2 materials – Vinyl or Static Cling – Another caveat is that it is usually White or Clear.
If you are going to be using a clear material, they you need to use as a backer color. Lets say you have a logo that is red and blue. And it's clear material
you stick in on the back of car window, now the clear takes on the colors of the car window. Imagine if you have tint? It doesnt stand out instead the decal just blends in . If you had a white backer(not flood white) then it would make it POP out.
Now the same applies for static cling decals. I see a lot of order for static cling decals and people place these on the inside of their car windows. Which is fine, but without the White their is no Pop!
See A Video Here!