Custom Shader makes sprite darker (unintended side effect)

Let me just start off by saying that my experience with this subject is very little indeed. I wrote with the help of online tutorials a custom shader to make certain colored pixels transparent. But an unintended side effect was that my sprites turn much darker:

Imgur

Is this a gamma issue or perhaps something to do with shadows I wonder?

*Edit - code now included

Shader "Custom/transparent-color" {

     Properties {
         _Color ("Color", Color) = (1,1,1,1)
         _TransparentColor ("Transparent Color", Color) = (1,1,1,1)
         _Threshold ("Threshhold", Float) = 0.1
         _MainTex ("Albedo (RGB)", 2D) = "white" {}
     }
     SubShader {
         Tags { "Queue"="Transparent" "RenderType"="Transparent" }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf Lambert alpha
 
         sampler2D _MainTex;
 
         struct Input {
             float2 uv_MainTex;
         };
 
         fixed4 _Color;
         fixed4 _TransparentColor;
         half _Threshold;
 
         void surf (Input IN, inout SurfaceOutput o) {
            
             half4 c = tex2D (_MainTex, IN.uv_MainTex);
             half4 output_col = c * _Color;
             half3 transparent_diff = c.xyz - _TransparentColor.xyz;
             half transparent_diff_squared = dot(transparent_diff,transparent_diff);
             
             if(transparent_diff_squared < _Threshold)
                 discard;
             
             o.Albedo = output_col.rgb;
             o.Alpha = output_col.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }

Because you’re multiplying your color from your tex2D by your Color variable, if you assign any Color other than white you’re going to get a darker color than your tex2D input as a result of the multiplication operation since the Color value is a number from 0-1. Try using a different blending formula for the Color and tex2D inputs to get the output you’re looking for.