You can read the chapter on Materials in the latest manual.
A material definition looks like this :-

material mat_smaragd
{
ambient_blue = 100;
ambient_green = 255;
ambient_red = 100;
diffuse_blue = 0;
diffuse_green = 100;
diffuse_blue = 0;
specular_blue = 255;
specular_green = 255;
specular_red = 255;
power = 10;
}

then it can be assigned to an entity like :-

...
my.material = mat_smaragd;
....