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

[Chaser_Code]

node.material.onBeforeCompile=(shader)=>{


shader.vertexShader=shader.vertexShader.replace(
"void main() {",
`varying float green;
void main() {`
)


shader.vertexShader=shader.vertexShader.replace(
"}",
`if(normal.y<0.0 && position.y<-0.2){ green=1.0; }
}`
)


shader.fragmentShader=shader.fragmentShader.replace(
"void main() {",
`varying float green;
void main() {`
)
shader.fragmentShader=shader.fragmentShader.replace(
"#include <dithering_fragment>",
`#include <dithering_fragment>
if(green==1.0){ gl_FragColor.rgb=vec3(0.0,1.0,0.0); }
`
)

};