Custom shader - Material doesn't have a color property '_Color'

I'm trying to make a custom shader to do some coloring and later transparency based on the rim (fake fresnel effects).

Thus far I have this:

 Shader "Custom/Fresnel" {
   Properties {
     _EdgeColor ("Edge Color", Color) = (1.0, 0.0, 1.0, 0.0)
     _Color ("Color", Color) = (0.0, 1.0, 0.0, 0.0)
   }
   SubShader {
     Tags { "RenderType" = "Opaque" }
     CGPROGRAM
     #pragma surface surf Lambert
     #pragma target 3.0
     
     samplerColor _EdgeColor;
     samplerColor _Color;
 
     struct Input {
       float3 viewDir;
     };
     
     void surf (Input IN, inout SurfaceOutput o) {
       half rim = dot(normalize(IN.viewDir), o.Normal);
       o.Albedo = _EdgeColor * rim + _Color * (1.0 - rim);
     }
     ENDCG
   }
   Fallback "Diffuse"
 }

The problem is I get the following errors:

 Material doesn't have a color property '_EdgeColor'
 UnityEditor.DockArea:OnGUI()
 Material doesn't have a color property '_Color'
 UnityEditor.DockArea:OnGUI()

I've made a new material, and set its shader to my custom shader, and two color pickers show up, one for each. Do I need to write a custom lighting model or something?

Thanks!