GL.Quads not correctly displaying color. It's all black

Hey I'm currently in a situation where I have several cameras in my scene, each with the TestClass script (se below) attached. Each of their textures (_texture) is being draw correctly on screen, but when I start adding elements in the Splatter dictionary the quads added are all black, regardless of what color I pass along. Why is that? I'm using a simple VertexLit shader.

What am I missing?

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using UnityEngine;
 using System.Diagnostics;
 
 [RequireComponent(typeof(Camera))]
 public class TestClass : MonoBehaviour
 {
     private Texture2D _texture;
     private Material _mat;
     private RenderTexture _renderTexture;
     public Dictionary<Vector2,Color> Splatter;
 
     void Awake()
     {
         Splatter = new Dictionary<Vector2, Color>();
     }
 
     public RenderTexture SetTexture(Texture2D tex, Material mat)
     {
         _mat = mat;
         _texture = tex;
         _renderTexture = new RenderTexture(tex.width,tex.height, 0);
         _renderTexture.generateMips = false;
         _renderTexture.filterMode = FilterMode.Point;
         _renderTexture.Create();
         Graphics.Blit(tex, _renderTexture);
         GetComponent<Camera>().targetTexture = _renderTexture;
         
         return _renderTexture;
     }
 
     public void OnPostRender()
     {
         Graphics.Blit(_texture, _renderTexture);
 
         GL.PushMatrix();
         
         GL.LoadPixelMatrix(0, _texture.width, _texture.height, 0);
 
         GL.Begin(GL.QUADS);
 
         foreach (var pos in Splatter)
         {
             GL.Color(pos.Value);
             GL.Vertex3(pos.Key.x, -pos.Key.y + _texture.height, 0);
             GL.Vertex3(pos.Key.x + 1, -pos.Key.y + _texture.height, 0);
             GL.Vertex3(pos.Key.x + 1, -pos.Key.y - 1 + _texture.height, 0);
             GL.Vertex3(pos.Key.x, -pos.Key.y - 1 + _texture.height, 0);
 
         } 
         GL.End();
         GL.PopMatrix();
     }
 }

I've done all of this as a way of manipulating a texture without using Texture2D.Apply().

Hope someone out there can help :)

Kind Regards Lasse