Geometry Shader beginner question (interpolation)

Hi everyone,

I'm jumping into the extra dimension of geometry shader...
There is some useful (and accessible) resources on the net like this one:
https://learnopengl.com/Advanced-OpenGL/Geometry-Shader
First step is to build houses based on points. Like this :

After translation from version 330 to version 120 (not an easy step...), I manage to have houses, but with a very different snow effect:

Inside the geometry shader, there is the function to create new vertice and change the color for the roof:

void build_house(vec4 position)
{
gl_FrontColor = gl_FrontColorIn[0];
gl_Position = position + vec4(-0.2, -0.2, 0.0, 0.0);
EmitVertex();
gl_Position = position + vec4( 0.2, -0.2, 0.0, 0.0);
EmitVertex();
gl_Position = position + vec4(-0.2, 0.2, 0.0, 0.0);
EmitVertex();
gl_Position = position + vec4( 0.2, 0.2, 0.0, 0.0);
EmitVertex();
gl_Position = position + vec4( 0.0, 0.4, 0.0, 0.0);
gl_FrontColor = vec4(1.0);
EmitVertex();
EndPrimitive();
}

What I understand is: for the first 4 vertices, color is related to the gl_FrontColor. For the last one, the color is white. Because of the shader system, the color has to interpolate from gl_FrontColor to white, is that right ?
What am I missing ?!