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:

http://imgur.com/9xFfhDa

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"
  }