title: Unity Shader RGBA Color mask
body: <p>I am working on a shader for Unity in which I want to change the color based on an mask image. In this mask image the RGB channels stand for a color that can be choosen in the shader. The idea behind the shader is that it is easy to change the look of an object without having to change the texture by hand.</p>
  
  <pre><code>Shader "Custom/MultiColor" {
      Properties {
          _MainTex ("Base (RGB)", 2D) = "white" {}
          _MaskTex ("Mask area (RGB)", 2D) = "black" {}
          _ColorR ("Red Color", Color) = (1,1,1,1)
          _ColorG ("Green Color", Color) = (1,1,1,1)
          _ColorB ("Blue Color", Color) = (1,1,1,1)
      }
      SubShader {
          Tags { "RenderType"="Opaque" }
          LOD 200
  
          CGPROGRAM
          #pragma surface surf Lambert
  
          sampler2D _MainTex;
          sampler2D _MaskTex;
          half4 _ColorR;
          half4 _ColorG;
          half4 _ColorB;
          half4 _MaskMult;
  
          struct Input {
              float2 uv_MainTex;
          };
  
          void surf (Input IN, inout SurfaceOutput o) {
              half4 main = tex2D (_MainTex, IN.uv_MainTex);
              half4 mask = tex2D (_MaskTex, IN.uv_MainTex);
  
              half3 cr = main.rgb * _ColorR;
              half3 cg = main.rgb * _ColorG;
              half3 cb = main.rgb * _ColorB;
  
              half r = mask.r;
              half g = mask.g;
              half b = mask.b;
  
              half minv = min(r + g + b, 1);
  
              half3 cf = lerp(lerp(cr, cg, g*(r+g)), cb, b*(r+g+b));
              half3 c = lerp(main.rgb, cf, minv);
  
              o.Albedo = c.rgb;
              o.Alpha = main.a;
          }
          ENDCG
      } 
      FallBack "Diffuse"
  }
  </code></pre>
  
  <p>The problem with the shader is the blending between the masked color based on the green and blue channel. Between colors defined in the color supposted to be from the red region is visible. Below a sample is visable. 
  <img src="https://i.stack.imgur.com/37G1Y.png" alt="enter image description here"></p>
  
  <p>The red color is create by the red mask, green by the green mask and desert yellow by the blue region. I do not know why this happens or how to solve this problem.</p>