I did as you said and it worked for the Decal but you are right, I will look into using an overlay texture on the final material it’s probably more appropriate.
Decals take lighting information from the object that they are placed upon, so the bright blue default main could be providing so much blue color into the lighting information for the decal.
You could try to up the value of the color information by multiplying by a constant. You may want to look at ultimately not using decals and using an overlay texture in your character’s final material controlled by a scalar variable which then can be set in your Character’s blueprint by the same key press.