unity shader 剔除指定的颜色

Shader "MyShader/PaintingBGTransparency" {
    Properties{
        _MainTex("Base (RGB)", 2D) = "white" {}
        _FilterfColor("Ridof (RGB)",Color) = (1,1,1,1)
    }
        SubShader{
            Tags { "RenderType" = "Opaque" }
            Blend SrcAlpha OneMinusSrcAlpha
            pass
            {
                CGPROGRAM
 
                #pragma vertex vertext_convert
                #pragma fragment fragment_convert
                #include "UnityCG.cginc"
 
                sampler2D  _MainTex;
                sampler2D  _MainTex1;
                float4  _FilterfColor;
                struct Inputvrite
                {
                    float4 vertex : POSITION;
                    float4 texcoord : TEXCOORD0;
                };
                struct Inputfragment
                {
                    float4 pos : SV_POSITION;
                    float4 uv : TEXCOORD0;
                };
 
                float ColorLerp(float3 tmp_nowcolor,float3 tmp_FilterfColor)
                {
                    float3 dis = float3(abs(tmp_nowcolor.x - tmp_FilterfColor.x),abs(tmp_nowcolor.y - tmp_FilterfColor.y),abs(tmp_nowcolor.z - tmp_FilterfColor.z));
                    float dis0 = sqrt(pow(dis.x,2) + pow(dis.y,2) + pow(dis.z,2));
                    float maxdis = sqrt(3);
                    float dis1 = lerp(0,maxdis,dis0);
                    return dis1;
                }
 
                Inputfragment vertext_convert(Inputvrite i)
                {
                    Inputfragment o;
                    o.pos = UnityObjectToClipPos(i.vertex);
                    o.uv = float4(i.texcoord.xy,1,1);
                    return o;
                }
 
                float4 fragment_convert(Inputfragment o) : COLOR
                {
                    float4 c = tex2D(_MainTex,o.uv);
                    c.a *= ColorLerp(c.rgb,_FilterfColor.rgb);
                    return c;
                }
 
 
            ENDCG
            }
        }
            FallBack "Diffuse"
}