It's invert color post process shader for Unity.(Use ImageEffect or uGUI's Image)

InvertColor.shader

Shader "Custom/InvertColor"
{
    // Propertiesはいらなくなる / Properties is unnecessary.

    SubShader {
        Tags {"Queue" = "Transparent+10000"} // RenderQueueは最後になるようにとにかくでかく / Set biggest value in your scene.

        GrabPass{} // 追加 / To add

        Pass {
            CGPROGRAM

            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

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

            struct v2f {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            v2f vert (appdata v) {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            sampler2D _GrabTexture; // _MainTexから変更 / Change from _MainTex

            fixed4 frag (v2f i) : SV_Target {
                fixed4 col = tex2D(_GrabTexture, i.uv);
		col.rgb = 1 - col.rgb;
                return col;
            }
            ENDCG
        }
    }
}

InvertColorImageEffect.shader

Shader "Custom/InvertColorImageEffect"
{
    Properties {
        _MainTex ("Texture", 2D) = "white" {}
    }

    SubShader {
        Pass {
            CGPROGRAM

            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

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

            struct v2f {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            v2f vert (appdata v) {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            sampler2D _MainTex;

            fixed4 frag (v2f i) : SV_Target {
                fixed4 col = tex2D(_MainTex, i.uv);
		col.rgb = 1 - col.rgb;
                return col;
            }
            ENDCG
        }
    }
}