1.颜色叠加
Blend SrcAlpha One

混合结果 = 源颜色的 alpha 通道 * 目标颜色的 alpha 通道 + 源颜色的颜色通道

2.shader代码

Shader "Custom/Blend2"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Alpha("Alpha",range(0,1)) = 1
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" }
        LOD 100

        Pass
        {
            Blend SrcAlpha One
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
                fixed4 vcol : COLOR;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                fixed4 fcol : COLOR;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;
            float _Alpha;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                o.fcol = v.vcol;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 col = fixed4(1,0,0,_Alpha);
                return col;
            }
            ENDCG
        }
    }
}

3.效果