on/off two tone lighting shader for Unity

Shader "Custom/On Off Lighting"
{
    Properties
    {
        _Color ("Color", Color) = (1,1,1,1)
        _Shadow ("Shadow Color", Color) = (1,1,1,1)
        _ShadowStrength ("Shadow Strength", Range(0,1)) = 0.2
    }
    SubShader {
        Tags { "RenderType" = "Opaque" }
        CGPROGRAM
        #pragma surface surf SimpleLambert

        fixed4 _Color;
        fixed4 _Shadow;
        float _ShadowStrength;

        half4 LightingSimpleLambert (SurfaceOutput s, half3 lightDir, half atten) {
            fixed4 shadow = lerp(_Color, _Shadow, _ShadowStrength);
            return lerp(shadow, _Color, atten);
        }

        struct Input {
            half null;
        };

        void surf (Input IN, inout SurfaceOutput o) {
            o.Albedo = 1;
        }
        ENDCG
    }
    Fallback "Diffuse"
}