How to check whether surface is within light/shadow? (i.e. for photosensitive shader)

Hello everyone!

I'm trying to make my shadows look like watercolour splats / ink-blots. So far I've written a modified version of the standard shader that can take a grayscale map and produce the ink effect, like so:


(ignore the fact that light/dark areas get inverted - that'll change)

For my next trick, I want to replace the user-supplied grayscale texture with unity's real-time shadow maps. The problem is that I have no idea how to access them / whether it's even possible to do so from within a surface shader.

For the sake of simplifying the problem - I would be crazy-grateful if anybody could show me how I would go about modifying the stripped down shader included below, such that shadowed areas are _ColorA and non-shadowed areas are _ColorB:

Spoiler: Stripped Down Shader Code

Code (csharp):

1.  
2. Shader "Custom/CustomShadow" {
3.     Properties {
4.         _ColorA ("Color A", Color) = (1,1,0,1)      
5.         _ColorB ("Color B", Color) = (0,1,1,1)
6.         _MainTex ("Albedo (RGB)", 2D) = "white" {}
7.         _Glossiness ("Smoothness", Range(0,1)) = 0.5
8.         _Metallic ("Metallic", Range(0,1)) = 0.0
9.     }
10.     SubShader {
11.         Tags { "RenderType"="Opaque" }
12.         LOD 200
13.        
14.         CGPROGRAM
15.         // Physically based Standard lighting model, and enable shadows on all light types
16.         #pragma surface surf Standard fullforwardshadows
17.  
18.         // Use shader model 3.0 target, to get nicer looking lighting
19.         #pragma target 3.0
20.  
21.         sampler2D _MainTex;
22.  
23.         struct Input {
24.             float2 uv_MainTex;
25.         };
26.  
27.         half _Glossiness;
28.         half _Metallic;
29.         fixed4 _ColorA;
30.         fixed4 _ColorB;
31.  
32.         void surf (Input IN, inout SurfaceOutputStandard o) {
33.             // Albedo comes from a texture tinted by color
34.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _ColorA;
35.             o.Albedo = c.rgb;
36.             // Metallic and smoothness come from slider variables
37.             o.Metallic = _Metallic;
38.             o.Smoothness = _Glossiness;
39.             o.Alpha = c.a;
40.         }
41.         ENDCG
42.     }
43.     FallBack "Diffuse"
44. }
45.  

If anybody can show me how to go about sampling shadows in an example like that, I'm pretty sure I can figure out the rest.

If there's no easy solution, I would appreciate any advice that might point me in the right direction.

Thanks for your time!

-Hadyn