Paint a transparent object at runtime

Hi guys!

I need to paint a texture on a transparent object at runtime.

On the wiki I've found a shader called "Dissolve with Texture" and, basically, I neeed to revert it.

 // simple "dissolving" shader by genericuser (radware.wordpress.com)
 // clips materials, using an image as guidance.
 // use clouds or random noise as the slice guide for best results.
   Shader "Custom Shaders/Dissolving" {
     Properties {
       _MainTex ("Texture (RGB)", 2D) = "white" {}
       _SliceGuide ("Slice Guide (RGB)", 2D) = "white" {}
       _SliceAmount ("Slice Amount", Range(0.0, 1.0)) = 0.5
     }
     SubShader {
       Tags { "RenderType" = "Opaque" }
       Cull Off
       CGPROGRAM
       //if you're not planning on using shadows, remove "addshadow" for better performance
       #pragma surface surf Lambert addshadow
       struct Input {
           float2 uv_MainTex;
           float2 uv_SliceGuide;
           float _SliceAmount;
       };
       sampler2D _MainTex;
       sampler2D _SliceGuide;
       float _SliceAmount;
       void surf (Input IN, inout SurfaceOutput o) {
           clip(tex2D (_SliceGuide, IN.uv_SliceGuide).rgb - _SliceAmount);
           o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
       }
       ENDCG
     } 
     Fallback "Diffuse"
   }

Using this shader, at runtime I can "remove" some pixels clicking on a specific area of a mesh, but I need the opposite behaviour, that means to add part of a texture on an alpha object.

Here is the code I'm using to click on the mesh and select the area that must be painted

     var hit : RaycastHit;
     if (!Physics.Raycast (Camera.main.ScreenPointToRay(Input.mousePosition), hit)) return;
     
     var renderer : Renderer = hit.collider.renderer;
     var meshCollider = hit.collider as MeshCollider;
     
     var tex : Texture2D = texture;
     var pixelUV = hit.textureCoord;
     pixelUV.x *= tex.width;
     pixelUV.y *= tex.height;
     
     tex.SetPixel(pixelUV.x, pixelUV.y, Color.black);
     tex.Apply();
 

Can someone help me? I tried to modify the shader but I was not able to make it work as I need