Overlay that creates only black and white underneath.

Because there really is a lack of image effect documentation and it is pretty simple, here's a basic image effect that does this.

First, here is a C# script that you can attach to your camera;

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 [ExecuteInEditMode]
 public class GrayscaleFilter : MonoBehaviour 
 {
     private Material mat;
 
     public Texture overlay;
 
     private void OnRenderImage (RenderTexture src, RenderTexture dest)
     {
         if (!mat)
             mat = new Material (Shader.Find ("Hidden/GSF"));
 
         mat.SetTexture ("_Mask", overlay);
 
         Graphics.Blit (src, dest, mat);
     }
 
     private void OnDisable ()
     {
         if (mat)
             DestroyImmediate (mat);
     }
 }

And here is the accompanying shader;

 Shader "Hidden/GSF"
 {
     Properties
     {
         _MainTex ("Texture", 2D) = "white" {}
         _Mask ("Overlay", 2D) = "white" {}
     }
     SubShader
     {
         // No culling or depth
         Cull Off ZWrite Off ZTest Always
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             
             #include "UnityCG.cginc"
             
             sampler2D _MainTex;
             sampler2D _Mask;
 
             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;
             }
 
             fixed4 frag (v2f i) : SV_Target
             {
                 fixed4 c = tex2D (_MainTex, i.uv);
                 fixed4 gs = (c.r + c.g + c.b) / 3;
                 return lerp (gs, c, tex2D (_Mask, i.uv).r);
             }
             ENDCG
         }
     }
 }

As you can see, the majority of the actual code is done in about 6 lines.

This might work:

1) Duplicate your camera and make it render above the other one.

2) Give your duped cam a grayscale image effect.

3) Hardest part: Give the grayscale camera some kind of mask so not everything is gray.

Another way:

1) Duplicate your camera.

2) Give your duped cam a grayscale image effect.

3) Render the grayscale camera to a render texture.

4) Overlay the rendertexture over your normal, color camera and use some kind of shader to get a round mask.

Thanks for your answers.

Namey5: I unfortunately can't get your solution to work. Either my explanation was too bad or I'm just too dump to use shaders. (I have no clue of shaders btw.) To eliminate the first possibility I'll describe my problem more accurate (I think): I'm making a 2D Game and I wan't my character to sometimes be surrounded by an color-consuming aura. Therefore I need some kind of overlay/graphic that i can attach to my character that will render everything underneath it in gray-scales. Also it should be possible regulate the gray-scale/color-intensity by the transparency of the overlay, like my black circular thing on the second picture tries to show. If I just don't understand how to use your shader than I would be thankful if you could explain it to me.

N00MKRAD: I agree that this should somehow work, but I also agree to Namey5 that it would be quite expensive this way.

Anyways: Thanks to you too and hopefully someone will enlighten me.

Best Regards Janis Hansen