How to make the face of the model facing down to use another color?

Questions
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18

Try this:

node.material.onBeforeCompile = (shader) => {
  shader.vertexShader = `
    varying vec3 vN;
    ${shader.vertexShader}
  `.replace(
    `#include <normal_vertex>`,
    `#include <normal_vertex>
    vN = normal;
    //vN = (modelMatrix * vec4(normal, 1.)).xyz;
    `
  );
  shader.fragmentShader = `
    varying vec3 vN;
    ${shader.fragmentShader}
  `.replace(
    `#include <dithering_fragment>`,
    `#include <dithering_fragment>
    if(vN.y<0.0){ gl_FragColor.rgb=vec3(0.0,1.0,0.0); }`
  );
};

Depends on your needs, use
vN = normal; - object’s space
изображение

or
vN = (modelMatrix * vec4(normal, 1.)).xyz; - global space
изображение