效果

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

Shader "UI/ImageGreyShader"
{
    Properties
    {
        [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
        _Color("Tint", Color) = (1,1,1,1)
    }

        SubShader
        {
            Tags
        {
            "Queue" = "Transparent"
            "IgnoreProjector" = "True"
            "RenderType" = "Transparent"
            "PreviewType" = "Plane"
            "CanUseSpriteAtlas" = "True"
        }
            // 源rgba*源a + 背景rgba*(1-源A值)   
            Blend SrcAlpha OneMinusSrcAlpha

            Pass
        {
            CGPROGRAM
    #pragma vertex vert     
    #pragma fragment frag     
    #include "UnityCG.cginc"     

            struct appdata_t
        {
            float4 vertex   : POSITION;
            float4 color    : COLOR;
            float2 texcoord : TEXCOORD0;
        };

        struct v2f
        {
            float4 vertex   : SV_POSITION;
            fixed4 color : COLOR;
            half2 texcoord  : TEXCOORD0;
        };

        sampler2D _MainTex;
        fixed4 _Color;

        v2f vert(appdata_t IN)
        {
            v2f OUT;
            OUT.vertex = UnityObjectToClipPos(IN.vertex);
            OUT.texcoord = IN.texcoord;
    #ifdef UNITY_HALF_TEXEL_OFFSET     
            OUT.vertex.xy -= (_ScreenParams.zw - 1.0);
    #endif     
            OUT.color = IN.color * _Color;
            return OUT;
        }

        fixed4 frag(v2f IN) : SV_Target
        {
            half4 color = tex2D(_MainTex, IN.texcoord) * IN.color;
            float grey = dot(color.rgb, fixed3(0.22, 0.707, 0.071));
            return half4(grey,grey,grey,color.a);
        }
            ENDCG
        }
        }
}