Material won't apply to model

so I made a robot 3dmodel and un unwraped most part of it, not every part. and designed a very sinple purple color texture to it’s head, to test if the texture works in UE4. Surprisingly after I create an material, a Texture sample node, and hit apply, the purple color doesn’t show up, only thing changed visually is the defaut gray color of the material become a bit light gray.
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I have disabled the default material, cleaned everthing
My uv is correct, except I imported the hole robot model, but only unwraped some part of it’s uvw, because some small parts like the screws are not needed to unwrap at all, I dont this this matters who knows?
my texture file is a 8 bit TGA file, and this purple base color is designed on top of the uvw template, and the uvwtemplate is hided before I save the file, in photoshop

I have also uploaded screenshots. After I finished edit my texture in PS, I save it as 8 bit file, because this is the only way I can save the file as TGA, I need TGA in my project. And the background becomes white, can anyone tell me why this is also? I suppose this is how it works.
If I solve this problem I will finished the texture design of every part on the uv.

So what went wrong? I have been reserching this for 3 days and no anwser found. it has stoped my project, I start to think about learn Unity 5 instead, but I dont want to give up. Anyone can help help please:)
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The problem may be in how you are doing this. You gave me an excuse to download 4.13 and test it out. I made a quick cube in blender, unwrapped it and painted over the uvs in photoshop. Exported the image as TGA and was able to build and apply the material. Could you share your fbx and tga files so I can look at those?

Hi , the problem is I used OBJ file instead of FBX file, thanks for reminding me! it is solved

Hi , the problem is I used OBJ file instead of FBX file, thanks for reminding me! it is solved

Hi , the problem is I used OBJ file instead of FBX file, thanks for reminding me! it is solved