Drawing onto the Texture2D of a sprite

So I did eventually figure this out, below is a small script that I put in my scene that takes a reference to an empty spriteRenderer. This code creates a new sprite out of a new Texture2d, and fills in a quarter of it with red.

 public class SpriteDrawing : MonoBehaviour 
 {
     public SpriteRenderer spriteRenderer;
     private Texture2D spriteTexture2D;
 
     void Start () 
     {
         this.spriteTexture2D = new Texture2D(512,512);
     
         Rect r = new Rect(0,0,512,512);
         Vector2 pivot = new Vector2(0,0);
         Sprite sprite = Sprite.Create(this.spriteTexture2D,r,pivot);
         this.spriteRenderer.sprite = sprite;
 
         StartCoroutine("delayedDraw");
     }
 
     IEnumerator delayedDraw()
     {
         yield return new WaitForSeconds(.5f);
         for(int x = 0; x <= 256; x++)
         {
             for(int y = 0; y <= 256; y++)
             {
                 this.spriteTexture2D.SetPixel(x,y,Color.red);
             }
         }
         this.spriteTexture2D.Apply();
     }
 }