How to set color to transparent parts?
what transparent parts ?
what type of material ?
via blueprint, code, at runtime, or just in editor?
have any screenshots?
more details will help answering it 
As you see, there is transparancy on this material.
I want to add color, so i can change it in script (paint the car model, not touching this texture).
As long as you connect the “Alpha” with the “Opacity” you’ll keep getting transparent areas. or do you mean, you need to have those transparent areas, and then another layer of color underneath them ?
-m
Yeah, i mean something like that.
This is my trunk model with this material. As you see, it’s transpparent on top. I need to add color, not invisibility.
This is my car trunk model. As you see i need to set color, not transparency. I need this to make car paint system.
Can i add 2nd material in Unreal to existing mesh?
best practice it to make this via UV. In Maya/Max assign 2 different materials for the polygons.
Patch A, will have the material with texture, and mapped with the UV
Patch B, will have material without need for UV.
In Unreal, all you need is to set parameter for Material B, to control the color.
-m
if the fbx itself have materials per face, that will be applied in the mesh inside Unreal.
I mean open the model in Maya/Max or wherever you made it, and then select those faces, and apply different material for them.
Re-Export to Unreal, and now you’ll have 2 materials to apply for the same mesh.
got that?