How to vary the color according to the elevation?

Question

I would like to achieve something like this :

However, I fail to get the Y coordinates (before projection) of a given point inside the Fragment Shader.

Here is my Fragment Shader :

precision mediump float;

vec4 blue = vec4(0.0, 0.0, 1.0, 1.0);
vec4 pink = vec4(1.0, 0.0, 1.0, 1.0);

void main() {
    float y_coordinates = ???
    gl_FragColor = mix(blue , pink , y_coordinates);
}

Should I send them from the Vertex Shader?