Assuming you were willing to just reduce the saturation of the colors a bit, I think you can use
LayeredRegion:SetVertexColor() to just directly modify the colors. I'm sure someone else here knows if I've got that right.
EDIT: You got the spelling right
. My curiosity got the better of me and I looked up the etymology.
Vignette means "little vines";
vigne is the French word for "vine".