Shader that changes color based on displacement from radius (height shader for deformed sphere).

I have create a GLSL shader that is supposed to find the vertex coordinates of a deformed sphere and compare that to the original radius to create a displacement value that can then be used to alter the color of a point based on its displacement from the normal. For some reason, in the following shader, displacement is always equal to zero and the object is white along all points of varying displacement. Is there anything that I am doing wrong and can fix to achieve the desired effect? Is it possible that I need to use Unity's coordinate system for the radius as the object is first created as a 0.5^3 cube and then normalized to make a sphere then scaled to the radius? (The Radius value is taken from the object by using renderer.material.SetFloat("_radius", radius) but can be changed manually.)

 Shader "Custom/SphereShader" {
 
 
     Properties
     {
     
         _radius("Radius", Float) = 50
         
     }
         
    SubShader {
       Pass {
          GLSLPROGRAM
  
          #ifdef VERTEX
   
          varying vec3 position;
          varying float displacement;
          uniform float _radius;
         varying float radius;
          varying float _length;
          varying vec4 _color;
          
             
  
          void main()
          {
              radius = _radius;
              position = gl_Vertex;
              _length = length(position);
              displacement = ((_length - radius)/radius);
              _color = ((1.0 - displacement), (displacement + 1.0), 1.0, 1.0);
             gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
             
          }
  
          #endif
  
          #ifdef FRAGMENT
  
          varying vec4 _color;
  
          void main()
          {
              //RGBA
             gl_FragColor = _color;
 
          }
  
          #endif
  
          ENDGLSL
       }
    }
 }