This is a simplified shader example (here I use colors instead of light/shadow effect and only 2 sides instead of 4):

Shader "Test/UnlitTest"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _TopColor ("TopColor", Color) = (1, 0, 0, 1)
        _BottomColor ("_BottomColor", Color) = (0, 0, 1, 1)
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        ZTest Off
        ZWrite Off
        Lighting Off
        Cull Off
        Fog { Mode Off }

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            sampler2D _MainTex;
            float4 _MainTex_ST;
            float4 _MainTex_TexelSize;
            fixed4 _TopColor, _BottomColor;            

            struct vertData
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;    
            };

            struct fragData
            {
                float2 uv : TEXCOORD0;
                float2 uvW : TEXCOORD1;
                float4 vertex : SV_POSITION;
            };

            fragData vert (vertData v)
            {
                fragData f;
                f.vertex = UnityObjectToClipPos(v.vertex);
                f.uv = TRANSFORM_TEX(v.uv, _MainTex);
                f.uvW = mul(unity_ObjectToWorld, f.uv);
                return f;
            }

            fixed isTransparentPixel(fixed2 uv)
            {
                return step(tex2D(_MainTex, uv).a, 0.0);
            }

            fixed isEdgePixel(fixed2 uv)
            {
                uv = mul(unity_WorldToObject, uv); // back to object space
                return isTransparentPixel(uv);
            }

            fixed4 frag (fragData f) : SV_Target
            {
                fixed isTransparent = isTransparentPixel(f.uv);
                clip(-isTransparent);
                
                fixed4 c = tex2D(_MainTex, f.uv);
                float2 borderSize = float2(3, 3); // border size in tex pixels
                float dy = borderSize.y * _MainTex_TexelSize.y;
                fixed3 borderColor = fixed3(0, 0, 0);
                float borderColorsCount = 0;
                float2 uvW = f.uvW; // UV in the world space

                // Top edge:
                fixed isEdge = isEdgePixel(uvW + float2(0, dy));
                borderColor += isEdge * _TopColor.rgb;
                borderColorsCount += isEdge;

                // Bottom edge:
                isEdge = isEdgePixel(uvW + float2(0, -dy));
                borderColor += isEdge * _BottomColor.rgb;
                borderColorsCount += isEdge;

                fixed noBorderColor = step(length(borderColor), 0);
                fixed hasBorderColor = 1 - noBorderColor;
                c.rgb = c.rgb * noBorderColor + hasBorderColor * borderColor / max(borderColorsCount, 1);
                return c;
            }
            ENDCG
        }
    }
}

It gets a semi-transparent image as a texture. The image is completely transparent at the edges and contains an opaque shape in the center.

The shader colors the top edges of the non-transparent shape in red and the bottom ones in yellow. It uses unity_WorldToObject & unity_ObjectToWorld to check the edges facing directions in the world space.