Landscape layer painting - Gray components?

Hi. I’m new to UE4 and I have THAT problem when you try to paint multiple materials in a Landscape and you get a null textured component instead.


I have been browsing the forums and googling for answers but I have got no real solutions. Some people say the problem is you are trying to assign too many layers to a Landscape component, but in my case that’s clearly not the problem since I’m using only 3 textures and just 2 layers.
This is my material setup:


What am I doing wrong? I have tried to solve this problem all day but I don’t know what to do. I get this problem even if I only use the single diffuse texture and no specular.

Thanks in advance.

Meh… I found the problem.
I was using a Vector4 for the solid color constant when I should have been using a Vector3.

I leave this for the record for other newbies who come here by googling.