Font Color changing shader [color changes based on what background color is]

I want to create an effect to make text easier to read on progress bars. I want an effect similar to this:

I know I need to use a shader, and then just check the background color, and output the color I want in my fragment program.

Here is my current shader that takes in an input color, and based on a threshold color (right now it is right in the middle of rgba values as I want) and then changes my output color to black or white.

 Shader "FontEffects/InvertColor" 
 {
     Properties
     {
             //grey
         _ColorThreshhold ("Color Threshhold", Color) = (0.5, 0.5, 0.5,1.0)
             //White
         _BelowThreshhold ("Below Threshhold Color", Color) = (1.0,1.0,1.0,1.0)
             //Black
         _AboveThreshhold ("Above Threshhold Color", Color) = (0.0,0.0,0.0,1.0)
         
         _InputColor ("Input Color", Color) = (0.0,0.0,0.0,1.0)
             //declare a color here to save background to, how do I do this?
     }
     
     SubShader
     {

             Pass
             {
              //save color of background sample here into a float4
             }

         Pass
         {
             CGPROGRAM
             
             //pragmas
             #pragma vertex vert
             #pragma fragment frag
             
             //user defined vars
             uniform float4 _ColorThreshhold;
             uniform float4 _BelowThreshhold;
             uniform float4 _AboveThreshhold;
             
             uniform float4 _InputColor;
             
             //base input structs
             struct vertexInput
             {
                 float4 vertex : POSITION;
                 float4 vColor : COLOR0;
             };
             
             struct vertexOutput
             {
                 float4 pos : SV_POSITION;
                 float4 vColor : COLOR;
             };
             //vertex function
             vertexOutput vert(vertexInput v)
             {
                 vertexOutput o;
                 o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                 return o;
             }
             
             
             
             //fragment function
             float4 frag(vertexOutput i) : COLOR
             {                    
                 float sum = _InputColor.r + _InputColor.g + _InputColor.b;
                 float tSum = _ColorThreshhold.r + _ColorThreshhold.g + _ColorThreshhold.b;
                 
                 if(sum >= tSum)
                 {
                     //above threshhold
                     return _AboveThreshhold;
                 }
                 else
                 {
                     //below threshhold
                     return _BelowThreshhold;
                 }
                   
             }
             
             ENDCG
         }
     }
     //comment out only for debug so errors make object pink
     //Fallback "Diffuse"
 }

I know I will need to code another pass to grab the color that is behind it (before I render the font) and then use that for the color. For that I believe I just put in the code to declare a pass before my main pass, and then save to a nonUnity exposed variable name. Then I use this in the second pass.

Is this correct? (I want to avoid using shaderlab, and stick to cg to learn cg and work on chips that dont support shaderlab.)

Another question: Is it better to use a half instead of a float4 for rgb values? (less bits to handle in the gpu)? Or are colors handled just fine as float4?