Material.Lerp() with different shaders problem!

Hi,

I'm trying to reach something similar to the Unity's hierarchy gray-scale search feature using Material.Lerp() to lerp between 2 materials, Material1 and Material2. Both materials have same textures but different shaders. Material1 uses the Standard shader and Material2 uses 'GrayScale' shader that turns the colors(without loosing textures) to gray-scale. When I use Material.Lerp() function it doesn't work! As I understand from the documentation that in order this function to work, I should use the same textures on both materials witch I do. I went though all the precess of learning shader scripting to be able to solve this! I created a custom shader that is very similar to the standard shader. When I use the custom shader on Material1 and the grey-scale shader on Material2 the Material.Lerp() works but it lerps from the color on Material1 to the color on Material2 overriding the gray-scale effect. I could not understand how it does this. I suppose that Material.Lerp() should lerp to the color that is provided by the shdader (the gray-scalse shader). in other words, the gray-scaled color, but it's not. Why it does that and why it doesn't work with the standard shader but works on my custom shader? Here's all custom shaders I use

 GrayScale.shader
 
 Shader "Custom/GrayScale" {
     Properties{
         _Color("Color", Color) = (1,1,1,1)
         _MainTex("Albedo (RGB)", 2D) = "white" {}
     _Glossiness("Smoothness", Range(0,1)) = 0.5
         _Metallic("Metallic", Range(0,1)) = 0.0
     }
         SubShader{
         Tags{ "RenderType" = "Opaque" }
         LOD 200
 
         CGPROGRAM
         // Physically based Standard lighting model, and enable shadows on all light types
 #pragma surface surf Standard fullforwardshadows
 
         // Use shader model 3.0 target, to get nicer looking lighting
 #pragma target 3.0
 
         sampler2D _MainTex;
 
     struct Input {
         float2 uv_MainTex;
     };
 
     half _Glossiness;
     half _Metallic;
     fixed4 _Color;
 
     void surf(Input IN, inout SurfaceOutputStandard o) {
         // Albedo comes from a texture tinted by color
         fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
         //o.Albedo = c.rgb;
         o.Albedo = (c.r + c.g + c.b)/2;
         // Metallic and smoothness come from slider variables
         o.Metallic = _Metallic;
         o.Smoothness = _Glossiness;
         o.Alpha = c.a;
     }
     ENDCG
     }
         FallBack "Diffuse"
 }
 

And this is the custom shader I created

 Shader "Custom/1" 
 {
     Properties 
     {
         _Color ("Color Tint", Color) = (1,1,1,1)
         _SpecColor("Specular Color", Color) = (1,1,1,1)
         _Shininess("Speq Power", Range(-1,10)) = 0.5
         _MainTex("Main Texture", 2D) = "white"{}
         _BumpMap("Normal Map", 2D) = "bump"{}
         _RimColor("Rim Color", Color) = (1,1,1,1)
         _RimPower("Rim Power", Range(0.0, 10.0)) = 3.0
             
     }
 
     SubShader 
     {
         Tags { "RenderType"="Opaque" }
         
         CGPROGRAM
         #pragma surface surf BlinnPhong
 
 
         struct Input 
         {
             float4 color : COLOR;
             float2 uv_MainTex;
             float2 uv_BumpMap;
             float3 viewDir;
         };
         
         sampler2D _MainTex;
         sampler2D _BumpMap;
         float4 _Color;
         float4 _RimColor;
         float _RimPower;
         float _Shininess;
 
         void surf(Input IN, inout SurfaceOutput o)
         {
             //Handle texture
             fixed4 tex = tex2D(_MainTex, IN.uv_MainTex);
             float3 bump = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
 
             //Apply normal map
             o.Normal = bump.rgb;
 
             //Apply diffuse texture
             o.Albedo = tex.rgb * _Color.rgb;
             
             //Specular
             o.Specular = _Shininess;
             o.Gloss = tex.a;
 
             //Rim lights
             half rim = 1.0 - saturate(dot(normalize(IN.viewDir), o.Normal));
             o.Emission = _RimColor.rgb * pow(rim, _RimPower);
 
         }
         ENDCG
     }
     FallBack "Diffuse"
 }
 

Any help is much appreciated.