On V-Ray 3 I don’t manage to put a color outside the decal. I did put a bitmap in the diffuse color, I uncheck the tile option, change the “no UVW color” and I didn’t get a result. Just black, with crop or place there is sometime the red color.
I chose to add a diffuse layer here but it can be any kind.
You drag your new layer below the decal layer. They get drawn in the same order that they appear here. There’s no limit (I think) to the number of layers you can have.
I chose a texture in my diffuse layer but you can select a simple diffuse color by clicking on the diffuse swatch.
I am quite sure it could be done with the “no UVW color”, this color appears with place instead of “whole texture” but tile has no effect ! V-ray 3 lack a good documentation I did write to chaos support.