Use cg shader to change text color based on previous color rendered

I want to make text that sits on top of a progress bar, and would like the font color to change to make it easily readable as the progress bar fills.

I want the text to invert color from black or white, depending on if the background color is light o dark (respectively). How can this be accomplished?

My current shader is based off the the example diffuse shader. I want to make the inversion using hardware, and not based on grabpass for mobile performance reasons.

 Shader  "FontEffects/BlendInvertWhite"  { 
     Properties  { 
         _MainTex  ( "Texture to Blend" ,  2D )  =  "White"  {} 
     } 
     SubShader  { 
         Tags  {} 
         Pass  { 
             Blend OneMinusDstColor Zero
             SetTexture  [ _MainTex ]  { combine texture } 
         } 
     } 
 }