Custom shader with unlit textures is too bright

I don't know the first thing about shaders, but I'm trying to write one that wil let me show only a specific part of a texture by using another texture specifying the area in black and white.

I got this working, but I can't get my unlit textures to be lighted as other unlit shaders do in Unity. I added the o.Emission = c.rgb, but now the result is too bright.

Also, Unity keeps complaining about "The shader wants normals". Any pointers on how to fix that are also much appreciated. Thanks.

 Shader "Custom/DiffTrans" {
 Properties {
     _MainTex ("Base (RGB)", 2D) = "white" {}
     _TransTex ("Transparent (A)", 2D) = "white" {}
 }
 SubShader {
     Tags { "IgnoreProjector"="True" "RenderType"="Transparent" }
     LOD 100
     Lighting OFF
     ZWrite Off

     CGPROGRAM
     #pragma surface surf Lambert alpha

     sampler2D _MainTex;
     sampler2D _TransTex;
     
     struct Input {
         float2 uv_MainTex;
         float2 uv_TransTex;
     };

     void surf (Input IN, inout SurfaceOutput o) {
         fixed4 c = tex2D (_MainTex, IN.uv_MainTex);
         fixed4 d = tex2D (_TransTex, IN.uv_TransTex);
         
         o.Albedo = c.rgb;
         o.Alpha = d.rgb;
         o.Emission = c.rgb;
     }
             
     ENDCG

 } 
 FallBack "Transparent/VertexLit"

}