When to apply phong shading in volume data?

Question

I have a volume dataset, that I can render(this was part of a homework) I have already implemented ray casting using front-to-back composition and I want to integrate phong-shading. For computing the gradient I use forward-difference-method.

I know I can estimate the gradient of a voxel texture coordinate with the forward difference method.

My question would be, since phong-shading returns a color and front-to-back raycasting also returns a color, when do I apply the shading to the color of the ray function? After I have finalized my new color for a specific ray or while computing the color from the ray hit? Maybe this piece of code helps to understand my problem. I have added comments on where I think I should implement phong shading.

void main(){
     if (raycast(t0,t1)) {

        float t = t0;
        for(float t =t0; t<t1;t+=stepsize){
              vec3 p = ray_o + ray_d * t;
              vec3 coord = worldToVolumeCoord(p, min_box, max_box);      
              float s = texture(tex_vol, coord).r;
              vec4 color = texture(tex_tf, s);
              if(color.a > 0){
                  // Front to back , when to apply shading?????
                  C = C + (1-A) * color.a * vec3(color);
                  A = A + (1-A) * color.a;
              }
             // vec3 Gradient = getGradient(tex_vol, coord, stepsize)
             // But how to apply Phong Shading?
        }
        // Apply phong here?
    }
}