Is there any way to get negative color in this shader?

I'm trying to get this shader to output negative colors, but I simply don't know how. I've written a simple surface shader that does what I want, but the thing is, I want to put the shader on an object without normals, and I want the shader to be as simple as possible (no surface shader).

Here's the Unlit Transparent Color Shader I currently have:

 Shader "Unlit/Unlit Transparent Color"
 {
     Properties
     {
         _Color ("Color Tint", Color) = (1,1,1,1)   
         _MainTex ("Base (RGB) Alpha (A)", 2D) = "white"
     }
 
     Category
     {
         Lighting Off
         ZWrite Off
         Cull back
         Blend SrcAlpha OneMinusSrcAlpha
         Tags {Queue=Transparent}
 
         SubShader
         {
              Pass
              {
                 SetTexture [_MainTex]
                 {
                     ConstantColor [_Color]
                     Combine Texture * constant
                 }
             }
         }
     }
 }

I just want the colors from the texture to be negative, is there any way to do this inside this shader?

Thanks in advance